cfg80211: Save the regulatory domain with a lock

Saving the regulatory domain while setting custom regulatory domain
was done while accessing a RCU protected pointer but without any
protection.

Fix this by using RTNL while accessing the pointer.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Reported-by:  <syzbot+27771d4abcd9b7a1f5d3@syzkaller.appspotmail.com>
Reported-by:  <syzbot+db4035751c56c0079282@syzkaller.appspotmail.com>
Reported-by: Hans de Goede <hdegoede@redhat.com>
Fixes: beee2469

 ("cfg80211: Save the regulatory domain when setting custom regulatory")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210105165657.613e9a876829.Ia38d27dbebea28bf9c56d70691d243186ede70e7@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>