af_unix: Set error only when needed in unix_stream_sendmsg().

We will introduce skb drop reason for AF_UNIX, then we need to
set an errno and a drop reason for each path.

Let's set an error only when it's needed in unix_stream_sendmsg().

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