bpf: sockmap state change warning fix

psock will uninitialized in default case we need to do the same psock lookup
and check as in other branch. Fixes compile warning below.

kernel/bpf/sockmap.c: In function ‘smap_state_change’:
kernel/bpf/sockmap.c:156:21: warning: ‘psock’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  struct smap_psock *psock;

Fixes: 174a79ff ("bpf: sockmap with sk redirect support")
Reported-by: NDavid Miller <davem@davemloft.net>
Signed-off-by: NJohn Fastabend <john.fastabend@gmail.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

kernel/bpf/sockmap.c

@@ -188,6 +188,9 @@ static void smap_state_change(struct sock *sk)
 			smap_release_sock(sk);
 		break;
 	default:
+		psock = smap_psock_sk(sk);
+		if (unlikely(!psock))
+			break;
 		smap_report_sk_error(psock, EPIPE);
 		break;
 	}