netfilter: nf_tables: fix scheduling-while-atomic splat

nf_tables_check_loops() can be called from rhashtable list
walk so cond_resched() cannot be used here.

Fixes: 81ea0106

 ("netfilter: nf_tables: add rescheduling points during loop detection walks")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>