ofproto-dpif: Ditch SLOW_MATCH slow path reason.

ofproto-dpif: Ditch SLOW_MATCH slow path reason.

Before this patch, datapath keys with ODP_FIT_TO_LITTLE, would be
assigned subfacets and installed in the kernel with a SLOW_MATCH
slow path reason.  This is problematic, because these flow keys
can't be reliable converted into a 'struct flow' thus breaking a
fundamental assumption of ofproto-dpif.  This patch circumvents the
issue by skipping facet creation for these flows altogether.  This
approach has the added benefit of simplifying the code for future
patches.

Signed-off-by: Ethan Jackson <ethan@nicira.com>