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

@@ -7025,36 +7025,35 @@ 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) {
+	while (1) {
+		used_bg = cluster->block_group;
+		if (!used_bg)
+			return NULL;
+
+		if (used_bg == block_group)
+			return used_bg;
+
+		btrfs_get_block_group(used_bg);
+
+		if (!delalloc)
+			return used_bg;
+
+		if (down_read_trylock(&used_bg->data_rwsem))
+			return used_bg;
+
+		spin_unlock(&cluster->refill_lock);
+
+		down_read(&used_bg->data_rwsem);
+
+		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);
 	}
-
-	used_bg = cluster->block_group;
-	if (!used_bg)
-		return NULL;
-
-	if (used_bg == block_group)
-		return used_bg;
-
-	btrfs_get_block_group(used_bg);
-
-	if (!delalloc)
-		return used_bg;
-
-	if (down_read_trylock(&used_bg->data_rwsem))
-		return used_bg;
-
-	spin_unlock(&cluster->refill_lock);
-	down_read(&used_bg->data_rwsem);
-	locked = true;
-	goto again;
 }
 
 static inline void