btrfs: add error handling for scrub_workers_get()

Although it is a rare case, we'd better free previous allocated
memory on error.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>