openvswitch: Change ovs_flow_tbl_lookup_xx() APIs

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: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>