headers, netfilter: Use kernel type names __u8, __u16, __u32

These types are guaranteed to be defined by <linux/types.h> for
both userland and kernel, unlike u_intN_t.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>