btrfs: fix compiling with CONFIG_BTRFS_DEBUG enabled.

Merging in the free space tree deleted a variable needed when
CONFIG_BTRFS_DEBUG=y

Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent-tree.c

@@ -543,11 +543,13 @@ static noinline void caching_thread(struct btrfs_work *work)
 	struct btrfs_block_group_cache *block_group;
 	struct btrfs_fs_info *fs_info;
 	struct btrfs_caching_control *caching_ctl;
+	struct btrfs_root *extent_root;
 	int ret;
 
 	caching_ctl = container_of(work, struct btrfs_caching_control, work);
 	block_group = caching_ctl->block_group;
 	fs_info = block_group->fs_info;
+	extent_root = fs_info->extent_root;
 
 	mutex_lock(&caching_ctl->mutex);
 	down_read(&fs_info->commit_root_sem);