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 1f78160ce1 ("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.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;