btrfs: factor out loop logic from btrfs_free_extra_devids

This prepares the code to switching seeds devices to a proper list.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -1036,28 +1036,21 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
 	return ERR_PTR(ret);
 }
 
-/*
- * After we have read the system tree and know devids belonging to
- * this filesystem, remove the device which does not belong there.
- */
-void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step)
+static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
+				      int step, struct btrfs_device **latest_dev)
 {
 	struct btrfs_device *device, *next;
-	struct btrfs_device *latest_dev = NULL;
 
-	mutex_lock(&uuid_mutex);
-again:
 	/* This is the initialized path, it is safe to release the devices. */
 	list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) {
-		if (test_bit(BTRFS_DEV_STATE_IN_FS_METADATA,
-							&device->dev_state)) {
+		if (test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state)) {
 			if (!test_bit(BTRFS_DEV_STATE_REPLACE_TGT,
 				      &device->dev_state) &&
 			    !test_bit(BTRFS_DEV_STATE_MISSING,
 				      &device->dev_state) &&
-			     (!latest_dev ||
-			      device->generation > latest_dev->generation)) {
-				latest_dev = device;
+			    (!*latest_dev ||
+			     device->generation > (*latest_dev)->generation)) {
+				*latest_dev = device;
 			}
 			continue;
 		}
@@ -1095,6 +1088,19 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step)
 		btrfs_free_device(device);
 	}
 
+}
+
+/*
+ * After we have read the system tree and know devids belonging to this
+ * filesystem, remove the device which does not belong there.
+ */
+void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step)
+{
+	struct btrfs_device *latest_dev = NULL;
+
+	mutex_lock(&uuid_mutex);
+again:
+	__btrfs_free_extra_devids(fs_devices, step, &latest_dev);
 	if (fs_devices->seed) {
 		fs_devices = fs_devices->seed;
 		goto again;