Fix injectpaymentonion fees

[Lagrang3]

Fixes issue #9006.

According to the description of injectpaymentonion this command simulates an incoming htlc from a peer.
The onion to us contains the information to deduce the next hop.
To be consistent with this this description, the command parameter amount_msat should correspond to the
amount the peer has provided to us and not the amount we should forward to our next peer.

The intention is deduced from this statement

	payload = onion_decode(tmpctx, rs, blinding,
			       cmd->ld->accept_extra_tlv_types,
			       *msat, *cltv,
			       &failtlvtype,
			       &failtlvpos,
			       &explanation);

msat is the amount_msat parameter from the RPC, therefore is interpreted as the amount_in.
We did not see this before because we were using auto.sourcefree layer.

[Lagrang3]

At this point da14a54 we don't need auto.sourcefree anymore in xpay.