netfilter: nf_tables: avoid softlockup warnings in nft_chain_validate

This reverts commit
314c82841602 ("netfilter: nf_tables: can't schedule in nft_chain_validate"):
Since commit a60a5abe19d6 ("netfilter: nf_tables: allow iter callbacks to sleep")
the iterator callback is invoked without rcu read lock held, so this
cond_resched() is now valid.

Signed-off-by: Florian Westphal <fw@strlen.de>