netns: ebtable: allow unprivileged users to operate ebtables

ebt_table is a private resource of netns, operating ebtables
in one netns will not affect other netns, we can allow the
creator user of userns and netns to change the ebtables.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>