cfg80211: warn if db.txt is empty with CONFIG_CFG80211_INTERNAL_REGDB

It has happened twice now where elaborate troubleshooting has
undergone on systems where CONFIG_CFG80211_INTERNAL_REGDB [0]
has been set but yet net/wireless/db.txt was not updated.

Despite the documentation on this it seems system integrators could
use some more help with this, so throw out a kernel warning at boot time
when their database is empty.

This does mean that the error-prone system integrator won't likely
realize the issue until they boot the machine but -- it does not seem
to make sense to enable a build bug breaking random build testing.

[0] http://wireless.kernel.org/en/developers/Regulatory/CRDA#CONFIG_CFG80211_INTERNAL_REGDB

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Youngsin Lee <youngsin@qualcomm.com>
Cc: Raja Mani <rmani@qca.qualcomm.com>
Cc: Senthil Kumar Balasubramanian <senthilb@qca.qualcomm.com>
Cc: Vipin Mehta <vipimeht@qca.qualcomm.com>
Cc: yahuan@qca.qualcomm.com
Cc: jjan@qca.qualcomm.com
Cc: vthiagar@qca.qualcomm.com
Cc: henrykim@qualcomm.com
Cc: jouni@qca.qualcomm.com
Cc: athiruve@qca.qualcomm.com
Cc: cjkim@qualcomm.com
Cc: philipk@qca.qualcomm.com
Cc: sunnykim@qualcomm.com
Cc: sskwak@qualcomm.com
Cc: kkim@qualcomm.com
Cc: mattbyun@qualcomm.com
Cc: ryanlee@qualcomm.com
Cc: simbap@qualcomm.com
Cc: krislee@qualcomm.com
Cc: conner@qualcomm.com
Cc: hojinkim@qualcomm.com
Cc: honglee@qualcomm.com
Cc: johnwkim@qualcomm.com
Cc: jinyong@qca.qualcomm.com
Cc: stable@vger.kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@frijolero.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/reg.c

@@ -388,7 +388,15 @@ static void reg_regdb_query(const char *alpha2)
 
 	schedule_work(&reg_regdb_work);
 }
+
+/* Feel free to add any other sanity checks here */
+static void reg_regdb_size_check(void)
+{
+	/* We should ideally BUILD_BUG_ON() but then random builds would fail */
+	WARN_ONCE(!reg_regdb_size, "db.txt is empty, you should update it...");
+}
 #else
+static inline void reg_regdb_size_check(void) {}
 static inline void reg_regdb_query(const char *alpha2) {}
 #endif /* CONFIG_CFG80211_INTERNAL_REGDB */
 
@@ -2322,6 +2330,8 @@ int __init regulatory_init(void)
 	spin_lock_init(&reg_requests_lock);
 	spin_lock_init(&reg_pending_beacons_lock);
 
+	reg_regdb_size_check();
+
 	cfg80211_regdomain = cfg80211_world_regdom;
 
 	user_alpha2[0] = '9';