af_unix: Split restart label in unix_dgram_sendmsg().

There are two paths jumping to the restart label in unix_dgram_sendmsg().

One requires another lookup and sk_filter(), but the other doesn't.

Let's split the label to make each flow more straightforward.

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