mptcp: fix spurious wake-up on under memory pressure

The wake-up condition currently implemented by mptcp_epollin_ready()
is wrong, as it could mark the MPTCP socket as readable even when
no data are present and the system is under memory pressure.

Explicitly check for some data being available in the receive queue.

Fixes: 5684ab1a ("mptcp: give rcvlowat some love")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250113-net-mptcp-connect-st-flakes-v1-2-0d986ee7b1b6@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>