mptcp: don't always assume copied data in mptcp_cleanup_rbuf()

mptcp: don't always assume copied data in mptcp_cleanup_rbuf()

commit 551844f26da2a9f76c0a698baaffa631d1178645 upstream.

Under some corner cases the MPTCP protocol can end-up invoking
mptcp_cleanup_rbuf() when no data has been copied, but such helper
assumes the opposite condition.

Explicitly drop such assumption and performs the costly call only
when strictly needed - before releasing the msk socket lock.

Fixes: fd8976790a6c ("mptcp: be careful on MPTCP-level ack.")
Cc: stable@vger.kernel.org
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/20241230-net-mptcp-rbuf-fixes-v1-2-8608af434ceb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflicts in this version, because commit 581302298524 ("mptcp: error
  out earlier on disconnect") has not been backported to this version,
  and there was no need to do so. The only conflict was in protocol.c,
  and easy to resolve: the context was different, but the same addition
  can still be made at the same spot in mptcp_recvmsg(). ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>