neighbour: Use nlmsg_payload in neightbl_valid_dump_info

Update neightbl_valid_dump_info 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-v2-2-3d90cb42c6af@debian.org


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