af_unix: Move !sunaddr case in unix_dgram_sendmsg().

When other is NULL in unix_dgram_sendmsg(), we check if sunaddr
is NULL before looking up a receiver socket.

There are three paths going through the check, but it's always
false for 2 out of the 3 paths: the first socket lookup and the
second 'goto restart'.

The condition can be true for the first 'goto restart' only when
SOCK_DEAD is flagged for the socket found with msg->msg_name.

Let's move the check to the single appropriate path.

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