BUG_ON cleanup for drivers/md/

This changes two if() BUG(); usages to BUG_ON(); so people
can disable it safely.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 38a0a57..cb82816 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -4438,8 +4438,7 @@
 {
  	mddev_t *mddev = inode->i_bdev->bd_disk->private_data;
 
-	if (!mddev)
-		BUG();
+	BUG_ON(!mddev);
 	mddev_put(mddev);
 
 	return 0;