staging: erofs: fix return type of erofs_workgroup_get

There exists a return type misuse (`int'->`bool') since all
users assume it fails if only return value != 0, let's fix
the return type to `int' instead of confusing `bool'.

No logic changes.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>