Phonet: fix potential use-after-free in pep_sock_close()

sk_common_release() might destroy our last reference to the socket.
So an extra temporary reference is needed during cleanup.

Signed-off-by: Rémi Denis-Courmont <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/phonet/pep.c

···
 	struct pep_sock *pn = pep_sk(sk);
 	int ifindex = 0;
 
+	sock_hold(sk); /* keep a reference after sk_common_release() */
 	sk_common_release(sk);
 
 	lock_sock(sk);
···
 
 	if (ifindex)
 		gprs_detach(sk);
+	sock_put(sk);
 }
 
 static int pep_wait_connreq(struct sock *sk, int noblock)