selftests/mm: use generic read_sysfs in thuge-gen test

As generic read_sysfs is available in vm_utils, let's use is in thuge-gen
test.

Link: https://lkml.kernel.org/r/20250611100106.1331197-1-pulehui@huaweicloud.com


Signed-off-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>