Btrfs: Refactor btrfs_lock_cluster() to kill compiler warning

fs/btrfs/extent-tree.c: In function ‘btrfs_lock_cluster’:
fs/btrfs/extent-tree.c:6399: warning: ‘used_bg’ may be used uninitialized in this function

  - Replace "again: ... goto again;" by standard C "while (1) { ... }",
  - Move block not processed during the first iteration of the loop to the
    end of the loop, which allows to kill the "locked" variable,
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-and-Tested-by: Miao Xie <miaox@cn.fujitsu.com>
[ the compilation warning has been fixed by other patch, now we want to
  clean up the function ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -7025,17 +7025,9 @@ btrfs_lock_cluster(struct btrfs_block_group_cache *block_group,
 		   int delalloc)
 {
 	struct btrfs_block_group_cache *used_bg = NULL;
-	bool locked = false;
-again:
-	spin_lock(&cluster->refill_lock);
-	if (locked) {
-		if (used_bg == cluster->block_group)
-			return used_bg;
-
-		up_read(&used_bg->data_rwsem);
-		btrfs_put_block_group(used_bg);
-	}
 
+	spin_lock(&cluster->refill_lock);
+	while (1) {
 		used_bg = cluster->block_group;
 		if (!used_bg)
 			return NULL;
@@ -7052,9 +7044,16 @@ btrfs_lock_cluster(struct btrfs_block_group_cache *block_group,
 			return used_bg;
 
 		spin_unlock(&cluster->refill_lock);
+
 		down_read(&used_bg->data_rwsem);
-	locked = true;
-	goto again;
+
+		spin_lock(&cluster->refill_lock);
+		if (used_bg == cluster->block_group)
+			return used_bg;
+
+		up_read(&used_bg->data_rwsem);
+		btrfs_put_block_group(used_bg);
+	}
 }
 
 static inline void