af_unix: Use msg->{msg_name,msg_namelen} in unix_dgram_sendmsg().

In unix_dgram_sendmsg(), we use a local variable sunaddr pointing
NULL or msg->msg_name based on msg->msg_namelen.

Let's remove sunaddr and simplify the usage.

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