netns: fdb: allow unprivileged users to add/del fdb entries

Right now,only ixgdb,macvlan,vxlan and bridge implement
fdb_add/fdb_del operations.

these operations only operate the private data of net
device. So allowing the unprivileged users who creates
the userns and netns to add/del fdb entries will do no
harm to other netns.

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