Btrfs: free btrfs_device in place

It's pointless to defer it to a kthread helper as we're not under a
special context.

For reference, commit 1f78160c ("Btrfs: using rcu lock in the reader
side of devices list") introduced RCU freeing for device structures.

Originally the blkdev_put was called from free_device and rcu_barrier had
to be called. This is no longer required, bdev and our device structures
are now freed separately.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ enhance changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -824,26 +824,16 @@ void btrfs_close_extra_devices(struct btrfs_fs_devices *fs_devices, int step)
 	mutex_unlock(&uuid_mutex);
 }
 
-static void __free_device(struct work_struct *work)
+static void free_device(struct rcu_head *head)
 {
 	struct btrfs_device *device;
 
-	device = container_of(work, struct btrfs_device, rcu_work);
+	device = container_of(head, struct btrfs_device, rcu);
 	rcu_string_free(device->name);
 	bio_put(device->flush_bio);
 	kfree(device);
 }
 
-static void free_device(struct rcu_head *head)
-{
-	struct btrfs_device *device;
-
-	device = container_of(head, struct btrfs_device, rcu);
-
-	INIT_WORK(&device->rcu_work, __free_device);
-	schedule_work(&device->rcu_work);
-}
-
 static void btrfs_close_bdev(struct btrfs_device *device)
 {
 	if (device->bdev && device->writeable) {

fs/btrfs/volumes.h

@@ -133,7 +133,6 @@ struct btrfs_device {
 
 	struct btrfs_work work;
 	struct rcu_head rcu;
-	struct work_struct rcu_work;
 
 	/* readahead state */
 	spinlock_t reada_lock;