mptcp: add receive queue awareness in tcp_rcv_space_adjust()

mptcp: add receive queue awareness in tcp_rcv_space_adjust()

This is the MPTCP counter-part of commit ea33537d8292 ("tcp: add receive
queue awareness in tcp_rcv_space_adjust()").

Prior to this commit:

  ESTAB 33165568 0      192.168.255.2:5201 192.168.255.1:53380 \
        skmem:(r33076416,rb33554432,t0,tb91136,f448,w0,o0,bl0,d0)

After:

  ESTAB 3279168 0      192.168.255.2:5201 192.168.255.1]:53042 \
        skmem:(r3190912,rb3719956,t0,tb91136,f1536,w0,o0,bl0,d0)

Same throughput.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260407-net-next-mptcp-reduce-rbuf-v2-2-0d1d135bf6f6@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>