lpm: use atomic store to avoid partial update

lpm: use atomic store to avoid partial update

Compiler could generate non-atomic stores for whole table entry
updating. This may cause incorrect nexthop to be returned, if
the byte with valid flag is updated prior to the byte with nexthop
is updated.
Besides, field by field updating of table entries follow
read-modify-write sequences. The operations are not atomic,
nor efficient. And could cause entries out of synchronization.

Changed to use atomic store to update whole table entry.

Suggested-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>
Suggested-by: Gavin Hu <gavin.hu@arm.com>
Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>