routing: always pass rtentry to add_route_flags().

add_route_flags() uses rt prefix data to lookup the the current
rtentry from the routing table. Update rib_add_route_px() to
always pass rtentry regardless of the op_flags.

Reported by: Stefan Grundmann <sg2342@googlemail.com>
Approved by: re(cperciva)
MFC after: 1 day

(cherry picked from commit a0aa160b6d261f67b29a25c7cc5e04668fb6faf7)
(cherry picked from commit 45ced2987df29a8587df2ad3c9ea0ddb1998aa16)