Btrfs: move finish_wait out of the loop

If we're still going to wait after schedule(), we don't have to do
finish_wait() to remove our %wait_queue_entry since prepare_to_wait()
won't add the same %wait_queue_entry twice.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -622,8 +622,8 @@ static void btrfs_wait_for_no_snapshotting_writes(struct btrfs_root *root)
 		if (writers)
 			schedule();
 
-		finish_wait(&root->subv_writers->wait, &wait);
 	} while (writers);
+	finish_wait(&root->subv_writers->wait, &wait);
 }
 
 static int create_snapshot(struct btrfs_root *root, struct inode *dir,