net: mctp: perform source address lookups when we populate our dst

net: mctp: perform source address lookups when we populate our dst

Rather than querying the output device for its address in
mctp_local_output, set up the source address when we're populating the
dst structure. If no address is assigned, use MCTP_ADDR_NULL.

This will allow us more flexibility when routing for NULL-source-eid
cases. For now though, we still reject a NULL source address in the
output path.

We need to update the tests a little, so that addresses are assigned
before we do the dst lookups.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20260331-dev-mctp-null-eids-v1-1-b4d047372eaf@codeconstruct.com.au
Signed-off-by: Paolo Abeni <pabeni@redhat.com>