xtables: Fix position of replaced rules in cache

xtables: Fix position of replaced rules in cache

When replacing a rule, the replacement was simply appended to the
chain's rule list. Instead, insert it where the rule it replaces was.

This also fixes for zero counters command to remove the old rule from
cache.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>