Revert "Btrfs: device_list_add() should not update list when mounted"

This reverts commit b96de000.

This commit is triggering failures to mount by subvolume id in some
configurations.  The main problem is how many different ways this
scanning function is used, both for scanning while mounted and
unmounted.  A proper cleanup is too big for late rcs.

For now, just revert the commit and we'll put a better fix into a later
merge window.
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/volumes.c

@@ -529,12 +529,12 @@ static noinline int device_list_add(const char *path,
 		 */
 
 		/*
-		 * As of now don't allow update to btrfs_fs_device through
-		 * the btrfs dev scan cli, after FS has been mounted.
+		 * For now, we do allow update to btrfs_fs_device through the
+		 * btrfs dev scan cli after FS has been mounted.  We're still
+		 * tracking a problem where systems fail mount by subvolume id
+		 * when we reject replacement on a mounted FS.
 		 */
-		if (fs_devices->opened) {
-			return -EBUSY;
-		} else {
+		if (!fs_devices->opened && found_transid < device->generation) {
 			/*
 			 * That is if the FS is _not_ mounted and if you
 			 * are here, that means there is more than one
@@ -542,8 +542,7 @@ static noinline int device_list_add(const char *path,
 			 * with larger generation number or the last-in if
 			 * generation are equal.
 			 */
-			if (found_transid < device->generation)
-				return -EEXIST;
+			return -EEXIST;
 		}
 
 		name = rcu_string_strdup(path, GFP_NOFS);