nl80211: check netlink protocol in socket release notification

A non-privileged user can create a netlink socket with the same port_id as
used by an existing open nl80211 netlink socket (e.g. as used by a hostapd
process) with a different protocol number.

Closing this socket will then lead to the notification going to nl80211's
socket release notification handler, and possibly cause an action such as
removing a virtual interface.

Fix this issue by checking that the netlink protocol is NETLINK_GENERIC.
Since generic netlink has no notifier chain of its own, we can't fix the
problem more generically.

Fixes: 026331c4

 ("cfg80211/mac80211: allow registering for and sending action frames")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Ivanov <dima@ubnt.com>
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>