khugepaged: remove meaningless !pte_present() check in khugepaged_scan_pmd()

khugepaged: remove meaningless !pte_present() check in khugepaged_scan_pmd()

We know it must meet the !is_swap_pte() and !pte_none() condition if we
reach here.  Since !is_swap_pte() indicates pte_none() or pte_present()
is met, it's guaranteed that pte must be present here.

Link: https://lkml.kernel.org/r/20210325135647.64106-4-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>