ip{6}tables-restore: fix breakage due to new locking approach

Since (93587a0 ip[6]tables: Add locking to prevent concurrent instances),
ip{6}tables-restore does not work anymore:

iptables-restore < x
Another app is currently holding the xtables lock. Perhaps you want to use the -w option?

do_command{6}(...) is called from ip{6}tables-restore for every iptables
command contained in the rule-set file. Thus, hitting the lock error
after the second command.

Fix it by bypassing the locking in the ip{6}tables-restore path.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>