ofproto-dpif: Do not give stats to rules bypassed by "drop" frag policy.

ofproto-dpif: Do not give stats to rules bypassed by "drop" frag policy.

When the OFPC_FRAG_DROP policy is in effect, IP fragments are supposed to
be dropped before they reach the flow table.  Open vSwitch properly dropped
IP fragments in this case, but still accounted them to the packet and byte
counters for the flow that they would have hit if the OFPC_FRAG_NX_MATCh
policy had been in effect.

Reported-by: love you <thunder.love07@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>