btrfs: alloc_reserved_file_extent trace point should use extent_root

Even though a separate root is passed in, we're still operating on the
extent root.  Let's use that for the trace point.
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -8105,7 +8105,8 @@ static int alloc_reserved_file_extent(struct btrfs_trans_handle *trans,
 			ins->objectid, ins->offset);
 		BUG();
 	}
-	trace_btrfs_reserved_extent_alloc(root, ins->objectid, ins->offset);
+	trace_btrfs_reserved_extent_alloc(fs_info->extent_root,
+					  ins->objectid, ins->offset);
 	return ret;
 }