mptcp: fix the incorrect judgment for msk->cb_flags

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: 65a569b03ca8 ("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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>