Revert "netfilter: avoid get_random_bytes calls"

This reverts commit a42b99a6.

Hannes Frederic Sowa reported some problems with this patch, more specifically
that prandom_u32() may not be ready at boot time, see:

http://marc.info/?l=linux-netdev&m=138896532403533&w=2



Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>