rib: fix traversal with /32 route

rib: fix traversal with /32 route

If a /32 route is entered in the RIB the code to traverse
will not see end of the tree. This is due to trying
to do a negative shift which is an undefined in C.

Fix by checking for max depth as is already done in rib6.

Fixes: 5a5793a5ffa2 ("rib: add RIB library")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>