mptcp: leverage the backlog for RX packet processing

mptcp: leverage the backlog for RX packet processing

When the msk socket is owned or the msk receive buffer is full,
move the incoming skbs in a msk level backlog list. This avoid
traversing the joined subflows and acquiring the subflow level
socket lock at reception time, improving the RX performances.

When processing the backlog, use the fwd alloc memory borrowed from
the incoming subflow. skbs exceeding the msk receive space are
not dropped; instead they are kept into the backlog until the receive
buffer is freed. Dropping packets already acked at the TCP level is
explicitly discouraged by the RFC and would corrupt the data stream
for fallback sockets.

Special care is needed to avoid adding skbs to the backlog of a closed
msk and to avoid leaving dangling references into the backlog
at subflow closing time.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251121-net-next-mptcp-memcg-backlog-imp-v1-14-1f34b6c1e0b1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>