mptcp: sched: check both backup in retrans

The 'mptcp_subflow_context' structure has two items related to the
backup flags:

 - 'backup': the subflow has been marked as backup by the other peer

 - 'request_bkup': the backup flag has been set by the host

Looking only at the 'backup' flag can make sense in some cases, but it
is not the behaviour of the default packet scheduler when selecting
paths.

As explained in the commit b6a66e52 ("mptcp: sched: check both
directions for backup"), the packet scheduler should look at both flags,
because that was the behaviour from the beginning: the 'backup' flag was
set by accident instead of the 'request_bkup' one. Now that the latter
has been fixed, get_retrans() needs to be adapted as well.

Fixes: b6a66e52 ("mptcp: sched: check both directions for backup")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240826-net-mptcp-close-extra-sf-fin-v1-3-905199fe1172@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -2326,7 +2326,7 @@ struct sock *mptcp_subflow_get_retrans(struct mptcp_sock *msk)
 			continue;
 		}
 
-		if (subflow->backup) {
+		if (subflow->backup || subflow->request_bkup) {
 			if (!backup)
 				backup = ssk;
 			continue;