netfilter: avoid get_random_bytes calls

All these users need an initial seed value for jhash, prandom is
perfectly fine.  This avoids draining the entropy pool where
its not strictly required.

nfnetlink_log did not use the random value at all.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>