net: mpls: fix memdup.cocci warning

Simply use kmemdup instead of explicitly allocating and copying memory.

Generated by: scripts/coccinelle/api/memdup.cocci

Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Link: https://lore.kernel.org/r/20220406114629.182833-1-gongruiqi1@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>