API changes only for code readability. No functional chnages.

This patch removes the underscored version. Added a new API
ovs_flow_tbl_lookup_stats() that returns the n_mask_hits.

Reported by: Ben Pfaff <blp@nicira.com>
Reviewed-by: NThomas Graf <tgraf@redhat.com>
Signed-off-by: NAndy Zhou <azhou@nicira.com>
Signed-off-by: NJesse Gross <jesse@nicira.com>