fstools: allow multiple mount points for a single device (e.g., Btrfs subvolumes)

[shenwii]

Summary

This MR enables support for mounting the same physical device (identified by UUID, Label, or device path) to multiple target mount points.

Background

Although the Linux kernel allows a single device to be mounted to different locations, the current implementation of fstools incorrectly treats the device identifier as a unique key, preventing multiple mount entries. This is particularly restrictive for Btrfs users who need to mount different subvolumes (subvol) to various system paths.

Key Changes

1. mount_add function
   Changed the internal storage key from a simple identifier (UUID/Label/Device) to a composite key: concat(identifier, ",", target). This allows the system to store and track multiple configuration records for the same device.

2. find_block & find_mount_point functions
   Updated these functions to return multiple device+target combinations. I have implemented a limit of 10 mount points per device, which should be sufficient for most use cases. All calling functions have been updated with loops to process these multiple results.

3. umount_device function
   Added a skip_extroot flag to handle logic conflicts between mount_action and main_umount.

Testing

Configuration Example

To demonstrate the use case, this change allows a single Btrfs partition to be mounted to multiple paths using subvolumes in /etc/config/fstab:

config mount
        option uuid 'a60604a0-11e2-447d-a6c2-58b85cc0533c'
        option target '/overlay'
        option enabled '1'
        option options 'subvol=rootfs'

config mount
        option uuid 'a60604a0-11e2-447d-a6c2-58b85cc0533c'
        option target '/media/main'
        option enabled '1'
        option options 'subvol=main'

config mount
        option uuid 'a60604a0-11e2-447d-a6c2-58b85cc0533c'
        option target '/media/test'
        option enabled '1'
        option options 'subvol=main'

Previously, if multiple mount sections shared the same UUID, the later entry would overwrite the previous one in the vlist (due to the device identifier being used as the unique key). This patch ensures each unique (device, target) pair is treated as a distinct entry.

Testing Result

The following commands have been tested and are working as expected:

• block mount
• block umount
• block detect
• block info

root@OpenWrt:~# mount | grep /dev/m
/dev/mmcblk0 on /overlay type btrfs (rw,relatime,ssd,discard=async,space_cache=v2,subvolid=351,subvol=/rootfs)
/dev/mmcblk0 on /media/main type btrfs (rw,relatime,ssd,discard=async,space_cache=v2,subvolid=256,subvol=/main)
/dev/mmcblk0 on /media/test type btrfs (rw,relatime,ssd,discard=async,space_cache=v2,subvolid=256,subvol=/main)

root@OpenWrt:~# block info
/dev/ubi0_6: UUID="4e39f331-3004-45b2-8bce-93c4f5b0268b" VERSION="w5r0" TYPE="ubifs"
/dev/mmcblk0: UUID="a60604a0-11e2-447d-a6c2-58b85cc0533c" MOUNT="/overlay" TYPE="btrfs"
/dev/mmcblk0: UUID="a60604a0-11e2-447d-a6c2-58b85cc0533c" MOUNT="/media/main" TYPE="btrfs"
/dev/mmcblk0: UUID="a60604a0-11e2-447d-a6c2-58b85cc0533c" MOUNT="/media/test" TYPE="btrfs"
/dev/fit0: UUID="4231ad45-78f64759-81a62cfb-4f81a93a" VERSION="4.0" MOUNT="/rom" TYPE="squashfs"

config 'global'
        option  anon_swap       '0'
        option  anon_mount      '0'
        option  auto_swap       '1'
        option  auto_mount      '1'
        option  delay_root      '5'
        option  check_fs        '0'

config 'mount'
        option  target  '/overlay'
        option  uuid    'a60604a0-11e2-447d-a6c2-58b85cc0533c'
        option  enabled '0'

config 'mount'
        option  target  '/media/main'
        option  uuid    'a60604a0-11e2-447d-a6c2-58b85cc0533c'
        option  enabled '0'

config 'mount'
        option  target  '/media/test'
        option  uuid    'a60604a0-11e2-447d-a6c2-58b85cc0533c'
        option  enabled '0'

config 'mount'
        option  target  '/rom'
        option  uuid    '4231ad45-78f64759-81a62cfb-4f81a93a'
        option  enabled '0'

Note: Currently, the block info command does not display mount options (such as Btrfs subvol). Consequently, while multiple mount points for the same device are now functional, the block info output may not yet reflect these specific options. I plan to submit a follow-up patch to include options details in the block info output to improve visibility.

Note: Changes in main_autofs and other areas were implemented for consistency but have not been fully tested in a live environment. Feedback or testing from others is welcome.

[shenwii]

#12308