af_unix: Clean up SOCK_DEAD error paths in unix_dgram_sendmsg().

When other has SOCK_DEAD in unix_dgram_sendmsg(), we hold
unix_state_lock() for the sender socket first.

However, we do not need it for sk->sk_type.

Let's move the lock down a bit.

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