mptcp: use mptcp_check_fallback helper

Use __mptcp_check_fallback() helper defined in net/mptcp/protocol.h,
instead of open-coding it in both __mptcp_do_fallback() and
mptcp_diag_fill_info().

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-5-db8f25f798eb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: geliangtang

net/mptcp/protocol.h

@@ -1065,7 +1065,7 @@ static inline bool mptcp_check_fallback(const struct sock *sk)
 
 static inline void __mptcp_do_fallback(struct mptcp_sock *msk)
 {
-	if (test_bit(MPTCP_FALLBACK_DONE, &msk->flags)) {
+	if (__mptcp_check_fallback(msk)) {
 		pr_debug("TCP fallback already done (msk=%p)", msk);
 		return;
 	}

net/mptcp/sockopt.c

@@ -916,7 +916,7 @@ void mptcp_diag_fill_info(struct mptcp_sock *msk, struct mptcp_info *info)
 			mptcp_pm_get_local_addr_max(msk);
 	}
 
-	if (test_bit(MPTCP_FALLBACK_DONE, &msk->flags))
+	if (__mptcp_check_fallback(msk))
 		flags |= MPTCP_INFO_FLAG_FALLBACK;
 	if (READ_ONCE(msk->can_ack))
 		flags |= MPTCP_INFO_FLAG_REMOTE_KEY_RECEIVED;