Fix double borrow of task when delivering user signals

[TroyNeubauer]

While experimenting with #237, I found a double borrow:

moss: Kernel panicked at src/arch/arm64/memory/fault.rs:46:20: Double mutable borrow of current task! Borrowed from: Some(Location { file: "src/arch/arm64/proc/signal.rs", line: 30, column: 16 })

Borrowing current_task over the copy_to_user in do_signal causes a double borrow when run_mem_fault_handler runs (since the user stack is demand-paged)

It would be nice to have the type system help us here, but looks like CurrentTaskGuard is already !Send + !Sync.

[hexagonal-sun]

Nice catch!

I presume the segfault tests you've added to usertest tripped the double borrow?

[TroyNeubauer]

Yeah - if you comment drop(signal); drop(task); you will trigger the double borrow

[hexagonal-sun]

Merged. Thanks for the contribution!