selftests: cgroup: fix unsigned comparison with less than zero

'size' is unsigned, it never less than zero.

Link: https://lkml.kernel.org/r/20221105110611.28920-1-yuehaibing@huawei.com


Fixes: 6c26df84 ("selftests: cgroup: return -errno from cg_read()/cg_write() on failure")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: zefan li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>