regulatory: fix reg_is_valid_request handling

There's a bug with the world regulatory domain, it
can be updated any time which is different from all
other regdomains that can only be updated once after
a request for them. Fix this by adding a check for
"processed" to the reg_is_valid_request() function
and clear that when doing a request.

While looking at this I also found another locking
bug, last_request is protected by the reg_mutex not
the cfg80211_mutex so the code in nl80211 is racy.
Remove that code as it only tries to prevent an
allocation in an error case, which isn't necessary.
Then the function can also become static and locking
in nl80211 can have a smaller scope.

Also change __set_regdom() to do the checks earlier
and not different for world/other regdomains.

Acked-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>