mptcp: call tcp_cleanup_rbuf on subflows

That is needed to let the subflows announce promptly when new
space is available in the receive buffer.

tcp_cleanup_rbuf() is currently a static function, drop the
scope modifier and add a declaration in the TCP header.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>