mptcp: schedule work for better snd subflow selection

Otherwise the packet scheduler policy will not be
enforced when pushing pending data at MPTCP-level
ack reception time.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>