netfilter: nft_compat: use call_rcu for nfnl_compat_get

Just use .call_rcu instead.  We can drop the rcu read lock
after obtaining a reference and re-acquire on return.

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