lpm: fix vector IPv4 lookup

lpm: fix vector IPv4 lookup

rte_lpm_lookupx4 could return wrong next hop when more than 256 tbl8
groups are created. This is caused by incorrect type casting of tbl8
group index that been stored in tbl24 entry. The casting caused group
index truncation and hence wrong tbl8 group been searched.

Issue fixed by applying proper mask to tbl24 entry to get tbl8 group index.

Fixes: dc81ebbacaeb ("lpm: extend IPv4 next hop field")
Fixes: cbc2f1dccfba ("lpm/arm: support NEON")
Fixes: d2cc7959342b ("lpm: add AltiVec for ppc64")
Cc: stable@dpdk.org
Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Tested-by: David Christensen <drc@linux.vnet.ibm.com>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>