mptcp: restore window probe

Since commit 72377ab ("mptcp: more conservative check for zero
probes") the MPTCP-level zero window probe check is always disabled, as
the TCP-level write queue always contains at least the newly allocated
skb.

Refine the relevant check tacking in account that the above condition
and that such skb can have zero length.

Fixes: 72377ab ("mptcp: more conservative check for zero probes")
Cc: stable@vger.kernel.org
Reported-by: Geliang Tang <geliang@kernel.org>
Closes: https://lore.kernel.org/d0a814c364e744ca6b836ccd5b6e9146882e8d42.camel@kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251028-net-mptcp-send-timeout-v1-3-38ffff5a9ec8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

net/mptcp/protocol.c

@@ -1299,7 +1299,12 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
 	if (copy == 0) {
 		u64 snd_una = READ_ONCE(msk->snd_una);
 
-		if (snd_una != msk->snd_nxt || tcp_write_queue_tail(ssk)) {
+		/* No need for zero probe if there are any data pending
+		 * either at the msk or ssk level; skb is the current write
+		 * queue tail and can be empty at this point.
+		 */
+		if (snd_una != msk->snd_nxt || skb->len ||
+		    skb != tcp_send_head(ssk)) {
 			tcp_remove_empty_skb(ssk);
 			return 0;
 		}