netfilter: use get_random_u32 instead of prandom

In the Linux kernel, the following vulnerability has been resolved:

netfilter: use get_random_u32 instead of prandom

bh might occur while updating per-cpu rnd_state from user context,

ie. local_out path.

BUG: using smp_processor_id() in preemptible [00000000] code: nginx/2725

caller is nft_ng_random_eval+0x24/0x54 [nft_numgen]

Call Trace:

check_preemption_disabled+0xde/0xe0

nft_ng_random_eval+0x24/0x54 [nft_numgen]

Use the random driver instead, this also avoids need for local prandom

state. Moreover, prandom now uses the random driver since d4150779e60f

("random32: use real rng for non-deterministic randomness").

Based on earlier patch from Pablo Neira.