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>

tools/testing/selftests/cgroup/cgroup_util.c

@@ -555,6 +555,7 @@ int proc_mount_contains(const char *option)
 ssize_t proc_read_text(int pid, bool thread, const char *item, char *buf, size_t size)
 {
 	char path[PATH_MAX];
+	ssize_t ret;
 
 	if (!pid)
 		snprintf(path, sizeof(path), "/proc/%s/%s",
@@ -562,8 +563,8 @@ ssize_t proc_read_text(int pid, bool thread, const char *item, char *buf, size_t
 	else
 		snprintf(path, sizeof(path), "/proc/%d/%s", pid, item);
 
-	size = read_text(path, buf, size);
-	return size < 0 ? -1 : size;
+	ret = read_text(path, buf, size);
+	return ret < 0 ? -1 : ret;
 }
 
 int proc_read_strstr(int pid, bool thread, const char *item, const char *needle)