ip6tables: Use consistent exit code for EAGAIN

ip6tables: Use consistent exit code for EAGAIN

As of commit 056564f6a (Add new exit value to indicate concurrency
issues), the IPv4 iptables binary returns exit status 4 to indicate that
the kernel returned EAGAIN when trying to update a table.  But ip6tables
still returns exit status 1 under the same circumstances.  Update
ip6tables to bring it in line with iptables behavior.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>