netfilter: ip6tables regression fix

Commit 78454473

 (netfilter: iptables: lock free counters) broke
ip6_tables by unconditionally returning ENOMEM in alloc_counters(),

Reported-by: Graham Murray <graham@gmurray.org.uk>
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>