netfilter: xtables: add scheduling opportunity in get_counters

There are reports about spurious softlockups during iptables-restore, a
backtrace i saw points at get_counters -- it uses a sequence lock and also
has unbounded restart loop.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>