netfilter: xt_physdev: Fix spurious error message in physdev_mt_check

Simplify the check in physdev_mt_check() to emit an error message
only when passed an invalid chain (ie, NF_INET_LOCAL_OUT).
This avoids cluttering up the log with errors against valid rules.

For large/heavily modified rulesets, current behavior can quickly
overwhelm the ring buffer, because this function gets called on
every change, regardless of the rule that was changed.

Signed-off-by: Todd Seidelmann <tseidelmann@linode.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>