cfg80211: regulatory: reject invalid hints

Reject invalid hints early in order to not cause a kernel
WARN later if they're restored to or similar.

Reported-by: syzbot+d451401ffd00a60677ee@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=d451401ffd00a60677ee
Link: https://lore.kernel.org/r/20200819084648.13956-1-johannes@sipsolutions.netSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/reg.c

@@ -2946,6 +2946,9 @@ int regulatory_hint_user(const char *alpha2,
 	if (WARN_ON(!alpha2))
 		return -EINVAL;
 
+	if (!is_world_regdom(alpha2) && !is_an_alpha2(alpha2))
+		return -EINVAL;
+
 	request = kzalloc(sizeof(struct regulatory_request), GFP_KERNEL);
 	if (!request)
 		return -ENOMEM;