mm/khugepaged: do not fail collapse_pte_mapped_thp() on SCAN_PMD_NULL

MADV_COLLAPSE on a file mapping behaves inconsistently depending on if PMD
page table is installed or not.

Consider following example:

p = mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
err = madvise(p, 2UL << 20, MADV_COLLAPSE);

fd is a populated tmpfs file.

The result depends on the address that the kernel returns on mmap(). If
it is located in an existing PMD table, the madvise() will succeed.
However, if the table does not exist, it will fail with -EINVAL.

This occurs because find_pmd_or_thp_or_none() returns SCAN_PMD_NULL when a
page table is missing, which causes collapse_pte_mapped_thp() to fail.

SCAN_PMD_NULL and SCAN_PMD_NONE should be treated the same in
collapse_pte_mapped_thp(): install the PMD leaf entry and allocate page
tables as needed.

Link: https://lkml.kernel.org/r/v5ivpub6z2n2uyemlnxgbilzs52ep4lrary7lm7o6axxoneb75@yfacfl5rkzeh
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Zach O'Keefe <zokeefe@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>