af_unix: Clean up error paths in unix_stream_sendmsg().

If we move send_sig() to the SEND_SHUTDOWN check before
the while loop, then we can reuse the same kfree_skb()
after the pipe_err_free label.

Let's gather the scattered kfree_skb()s in error paths.

While at it, some style issues are fixed, and the pipe_err_free
label is renamed to out_pipe to match other label names.

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