IB/rxe: Fix destination cache for IPv6

To successfully match an IPv6 path, the path cookie must match. Store it
in the QP so that the IPv6 path can be reused.

Replace open-coded version of dst_check() with the actual call, fixing the
logic. The open-coded version skips the check call if dst->obsolete is 0
(DST_OBSOLETE_NONE), proceeding to replace the route. DST_OBSOLETE_NONE
means that the route may continue to be used, though.

Fixes: 4ed6ad1e ("IB/rxe: Cache dst in QP instead of getting it...")
Signed-off-by: Andrew Boyer <andrew.boyer@dell.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>