netns: bridge: allow unprivileged users add/delete mdb entry

since the mdb table is belong to bridge device,and the
bridge device can only be seen in one netns.
So it's safe to allow unprivileged user which is the
creator of userns and netns to modify the mdb table.

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