Return -ENOMEM if alloc_workqueue() fails.  Don't return success.

Fixes: d8a650adf429 ("erofs: add per-cpu threads for decompression as an option")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/Y+4d0FRsUq8jPoOu@kiliSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>