mac80211: disallow moving netns

mac80211 currently assumes init_net for all interfaces,
so really will not cope well with network namespaces,
at least at this time.

To change this, we would have keep track of the netns
in addition to the ifindex, which is not something I
want to think about right now.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>