mpls: Use nlmsg_payload in mpls_valid_getroute_req

Leverage the new nlmsg_payload() helper to avoid checking for message
size and then reading the nlmsg data.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250414-nlmsg-v2-8-3d90cb42c6af@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>