netfilter: x_tables: make xt_replace_table wait until old rules are not used anymore

xt_replace_table relies on table replacement counter retrieval (which
uses xt_recseq to synchronize pcpu counters).

This is fine, however with large rule set get_counters() can take
a very long time -- it needs to synchronize all counters because
it has to assume concurrent modifications can occur.

Make xt_replace_table synchronize by itself by waiting until all cpus
had an even seqcount.

This allows a followup patch to copy the counters of the old ruleset
without any synchonization after xt_replace_table has completed.

Cc: Dan Williams <dcbw@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>