mm/khugepaged: use vma_is_anonymous

Clean up the vma->vm_ops usage.  Use vma_is_anonymous instead of
vma->vm_ops to make it more understandable.

Link: https://lkml.kernel.org/r/20220424071642.3234971-1-xu.xin16@zte.com.cn
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>