netlink: create helpers for checking type is an int

There's a number of policies which check if type is a uint or sint.
Factor the checking against the list of value sizes to a helper
for easier reuse.

v2: - new patch

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>