af_unix: Reuse out_pipe label in unix_stream_sendmsg().

This is a follow-up of commit d460b04b ("af_unix: Clean up
error paths in unix_stream_sendmsg().").

If we initialise skb with NULL in unix_stream_sendmsg(), we can
reuse the existing out_pipe label for the SEND_SHUTDOWN check.

Let's rename it and adjust the existing label as out_pipe_lock.

While at it, size and data_len are moved to the while loop scope.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250116053441.5758-9-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>