netfilter: xt_LOG: add net namespace support for xt_LOG

Add pernet support to xt_LOG by means of the new nf_log_set
function added in (30e0c6a6 netfilter: nf_log: prepare net
namespace support for loggers).

Since syslog ns has yet not been implemented, we don't want
the containers to DDOS host's syslogd. So only enable ebt_log
only from init_net and wait for syslog ns support

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>