datapath: conntrack: mark expected switch fall-through

datapath: conntrack: mark expected switch fall-through

Upstream commit:
    commit 279badc2a85be83e0187b8c566e3b476b76a87a2
    Author: Gustavo A. R. Silva <garsilva@embeddedor.com>
    Date:   Thu Oct 19 12:55:03 2017 -0500

    openvswitch: conntrack: mark expected switch fall-through

    In preparation to enabling -Wimplicit-fallthrough, mark switch cases
    where we are expecting to fall through.

    Notice that in this particular case I placed a "fall through" comment on
    its own line, which is what GCC is expecting to find.

    Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>