mac80211_hwsim: Allow wmediumd to attach to radios created in its netns

Registering wmediumd is currently limited to the initial network
namespace. This patch enables wmediumd to attach from non-initial
network namespaces using a user namespace having CAP_NET_ADMIN. A
registered wmediumd can forward frames on radios that have been created
in the same network namespace, even if they have been moved to other
network namespaces.

The wmediumd Netlink portid is tracked per net namespace. Additionally,
the portid is stored on all radios created in that net namespace to
simplify the portid lookup in the data path.

Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>