erofs: correct the remaining shrink objects

The remaining count should not include successful
shrink attempts.

Fixes: e7e9a307be9d ("staging: erofs: introduce workstation for decompression")
Cc: <stable@vger.kernel.org> # 4.19+
Link: https://lore.kernel.org/r/20200226081008.86348-1-gaoxiang25@huawei.com
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>