mptcp: fix double-unlock in mptcp_poll

mptcp_connect/28740 is trying to release lock (sk_lock-AF_INET) at:
[<ffffffff82c15869>] mptcp_poll+0xb9/0x550
but there are no more locks to release!
Call Trace:
lock_release+0x50f/0x750
release_sock+0x171/0x1b0
mptcp_poll+0xb9/0x550
sock_poll+0x157/0x470
? get_net_ns+0xb0/0xb0
do_sys_poll+0x63c/0xdd0

Problem is that __mptcp_tcp_fallback() releases the mptcp socket lock,
but after recent change it doesn't do this in all of its return paths.

To fix this, remove the unlock from __mptcp_tcp_fallback() and
always do the unlock in the caller.

Also add a small comment as to why we have this
__mptcp_needs_tcp_fallback().

Fixes: 0b4f33def7bbde ("mptcp: fix tcp fallback crash")
Reported-by: [e-mail]
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/mptcp/protocol.c

···
 	if (likely(!__mptcp_needs_tcp_fallback(msk)))
 		return NULL;
 
-	if (msk->subflow) {
-		release_sock((struct sock *)msk);
-		return msk->subflow;
-	}
-
-	return NULL;
+	return msk->subflow;
 }
 
 static bool __mptcp_can_create_subflow(const struct mptcp_sock *msk)
···
 			goto out;
 	}
 
+fallback:
 	ssock = __mptcp_tcp_fallback(msk);
 	if (unlikely(ssock)) {
-fallback:
+		release_sock(sk);
 		pr_debug("fallback passthrough");
 		ret = sock_sendmsg(ssock, msg);
 		return ret >= 0 ? ret + copied : (copied ? copied : ret);
···
 		if (ret < 0)
 			break;
 		if (ret == 0 && unlikely(__mptcp_needs_tcp_fallback(msk))) {
+			/* Can happen for passive sockets:
+			 * 3WHS negotiated MPTCP, but first packet after is
+			 * plain TCP (e.g. due to middlebox filtering unknown
+			 * options).
+			 *
+			 * Fall back to TCP.
+			 */
 			release_sock(ssk);
-			ssock = __mptcp_tcp_fallback(msk);
 			goto fallback;
 		}
 
···
 	ssock = __mptcp_tcp_fallback(msk);
 	if (unlikely(ssock)) {
 fallback:
+		release_sock(sk);
 		pr_debug("fallback-read subflow=%p",
 			 mptcp_subflow_ctx(ssock->sk));
 		copied = sock_recvmsg(ssock, msg, flags);
···
 	 */
 	lock_sock(sk);
 	ssock = __mptcp_tcp_fallback(msk);
+	release_sock(sk);
 	if (ssock)
 		return tcp_setsockopt(ssock->sk, level, optname, optval,
 				      optlen);
-
-	release_sock(sk);
 
 	return -EOPNOTSUPP;
 }
···
 	 */
 	lock_sock(sk);
 	ssock = __mptcp_tcp_fallback(msk);
+	release_sock(sk);
 	if (ssock)
 		return tcp_getsockopt(ssock->sk, level, optname, optval,
 				      option);
-
-	release_sock(sk);
 
 	return -EOPNOTSUPP;
 }