nl80211: use RCU to read regdom in reg get/dump

Use RCU here to read the regdomain, this will allow us
to remove the RTNL locking from the setter.

Note in nl80211_get_reg_do() we still need the RTNL to
do the wiphy lookup.

Link: https://lore.kernel.org/r/20220214101820.5d4acbcf2a46.Ibfc91980439862125e983d9adeebaba73fe38e2d@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>