Btrfs: fix wrong block group in trace during the free space allocation

We allocate the free space from the former block group, not the current
one, so should use the former one to output the trace information.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/extent-tree.c

@@ -6323,7 +6323,8 @@ static noinline int find_free_extent(struct btrfs_root *orig_root,
 				/* we have a block, we're done */
 				spin_unlock(&last_ptr->refill_lock);
 				trace_btrfs_reserve_extent_cluster(root,
-					block_group, search_start, num_bytes);
+						used_block_group,
+						search_start, num_bytes);
 				if (used_block_group != block_group) {
 					btrfs_put_block_group(block_group);
 					block_group = used_block_group;