af_unix: Clean up error paths in unix_dgram_sendmsg().

The error path is complicated in unix_dgram_sendmsg() because there
are two timings when other could be non-NULL: when it's fetched from
unix_peer_get() and when it's looked up by unix_find_other().

Let's move unix_peer_get() to the else branch for unix_find_other()
and clean up the error paths.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>