net/neighbor: Update neightbl_dump_info for strict data checking

Update neightbl_dump_info for strict data checking. If the flag is set,
the dump request is expected to have an ndtmsg struct as the header.
All elements of the struct are expected to be 0 and no attributes can
be appended.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David S. Miller <davem@davemloft.net>