staging: erofs: tidy up utils.c

Keep in line with erofs-outofstaging patchset:
 - Update comments in erofs_try_to_release_workgroup;
 - code style cleanup.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-21-gaoxiang25@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>