btrfs: introduce free_device helper

A helper to free a device and all it's dynamically allocated members,
like the rcu_string name or flush_bio. This is going to replace all
open coded places.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -180,6 +180,13 @@ static struct btrfs_fs_devices *alloc_fs_devices(const u8 *fsid)
 	return fs_devs;
 }
 
+static void free_device(struct btrfs_device *device)
+{
+	rcu_string_free(device->name);
+	bio_put(device->flush_bio);
+	kfree(device);
+}
+
 static void free_fs_devices(struct btrfs_fs_devices *fs_devices)
 {
 	struct btrfs_device *device;
@@ -220,6 +227,11 @@ void btrfs_cleanup_fs_uuids(void)
 	}
 }
 
+/*
+ * Returns a pointer to a new btrfs_device on success; ERR_PTR() on error.
+ * Returned struct is not linked onto any lists and must be destroyed using
+ * free_device.
+ */
 static struct btrfs_device *__alloc_device(void)
 {
 	struct btrfs_device *dev;
@@ -6257,8 +6269,8 @@ static struct btrfs_device *add_missing_dev(struct btrfs_fs_devices *fs_devices,
  *		is generated.
  *
  * Return: a pointer to a new &struct btrfs_device on success; ERR_PTR()
- * on error.  Returned struct is not linked onto any lists and can be
- * destroyed with kfree() right away.
+ * on error.  Returned struct is not linked onto any lists and must be
+ * destroyed with free_device.
  */
 struct btrfs_device *btrfs_alloc_device(struct btrfs_fs_info *fs_info,
 					const u64 *devid,