btrfs: Cleanup the "_struct" suffix in btrfs_workequeue

Since the "_struct" suffix is mainly used for distinguish the differnt
btrfs_work between the original and the newly created one,
there is no need using the suffix since all btrfs_workers are changed
into btrfs_workqueue.

Also this patch fixed some codes whose code style is changed due to the
too long "_struct" suffix.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Tested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 5d9a037..80754f9 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -95,7 +95,7 @@
 	/* per-device scrub information */
 	struct scrub_ctx *scrub_device;
 
-	struct btrfs_work_struct work;
+	struct btrfs_work work;
 	struct rcu_head rcu;
 	struct work_struct rcu_work;