dpif: Shift ufid support checking up to dpif_backer.

dpif: Shift ufid support checking up to dpif_backer.

Previously, the dpif layer was responsible for determining datapath
support for UFIDs, which resulted in all ovs-dpctl utilities
inserting/deleting flows from the datapath each time they are run.
Shift this responsibility up to the dpif_backer.

There are two users of this functionality: Revalidators check for UFID
support to request a terser dump using UFIDs, and dpif-netlink uses this
to request flow_del operations to only return the UFID/stats. The latter
case was previously hidden from revalidators, but this change makes them
aware of it, and reuses the same "udpif->enable_ufid" flag for reducing
overhead of both flow dump and flow delete.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>