btrfs: restructure find_free_dev_extent()

- make it return the start position and length of the max free space when it can
  not find a suitable free space.
- make it more readability

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 3c71d95..1e1c9a1 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -8099,7 +8099,7 @@
 	mutex_lock(&root->fs_info->chunk_mutex);
 	list_for_each_entry(device, &fs_devices->alloc_list, dev_alloc_list) {
 		u64 min_free = btrfs_block_group_used(&block_group->item);
-		u64 dev_offset, max_avail;
+		u64 dev_offset;
 
 		/*
 		 * check to make sure we can actually find a chunk with enough
@@ -8107,7 +8107,7 @@
 		 */
 		if (device->total_bytes > device->bytes_used + min_free) {
 			ret = find_free_dev_extent(NULL, device, min_free,
-						   &dev_offset, &max_avail);
+						   &dev_offset, NULL);
 			if (!ret)
 				break;
 			ret = -1;