neighbour: Use nlmsg_payload in neigh_valid_get_req

Update neigh_valid_get_req function to utilize the new nlmsg_payload()
helper function.

This change improves code clarity and safety by ensuring that the
Netlink message payload is properly validated before accessing its data.

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


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