mptcp: fix the incorrect judgment for msk->cb_flags

Coccicheck reports the error below:
net/mptcp/protocol.c:3330:15-28: ERROR: test of a variable/field address

Since the address of msk->cb_flags is used in __test_and_clear_bit, the
address should not be NULL. The judgment for if (unlikely(msk->cb_flags))
will always be true, we should check the real value of msk->cb_flags here.

Fixes: 65a569b0 ("mptcp: optimize release_cb for the common case")
Signed-off-by: Xiang Yang <xiangyang3@huawei.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230803072438.1847500-1-xiangyang3@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -3328,7 +3328,7 @@ static void mptcp_release_cb(struct sock *sk)
 
 	if (__test_and_clear_bit(MPTCP_CLEAN_UNA, &msk->cb_flags))
 		__mptcp_clean_una_wakeup(sk);
-	if (unlikely(&msk->cb_flags)) {
+	if (unlikely(msk->cb_flags)) {
 		/* be sure to set the current sk state before tacking actions
 		 * depending on sk_state, that is processing MPTCP_ERROR_REPORT
 		 */