netfilter ebtables: fix xt_AUDIT to work with ebtables

Even though ebtables uses xtables it still requires targets to
return EBT_CONTINUE instead of XT_CONTINUE. This prevented
xt_AUDIT to work as ebt module.

Upon Jan's suggestion, use a separate struct xt_target for
NFPROTO_BRIDGE having its own target callback returning
EBT_CONTINUE instead of cloning the module.

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>