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>

net/ipv6/netfilter/ip6_tables.c

@@ -795,6 +795,7 @@ get_counters(const struct xt_table_info *t,
 
 			ADD_COUNTER(counters[i], bcnt, pcnt);
 			++i;
+			cond_resched();
 		}
 	}
 }