netlink: don't crash if prefix for < byte is requested

netlink: don't crash if prefix for < byte is requested

If prefix is used with a datatype that has less than 8 bits an
assertion is triggered:

src/netlink.c:243: netlink_gen_raw_data: Assertion `len > 0' failed.

This is esoteric, the alternative would be to restrict prefixes
to ipv4/ipv6 addresses.

Simpler fix is to use round_up instead of divide.

Signed-off-by: Florian Westphal <fw@strlen.de>