selftests/mm: extract read_sysfs and write_sysfs into vm_util

Extract read_sysfs and write_sysfs into vm_util.  Meanwhile, rename the
function in thuge-gen that has the same name as read_sysfs.

Link: https://lkml.kernel.org/r/20250529155650.4017699-4-pulehui@huaweicloud.com


Signed-off-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>