mempolicy: migration attempt to match interleave nodes

Improve alloc_migration_target_by_mpol()'s treatment of MPOL_INTERLEAVE.

Make an effort in do_mbind(), to identify the correct interleave index for
the first page to be migrated, so that it and all subsequent pages from
the same vma will be targeted to precisely their intended nodes.  Pages
from following vmas will still be interleaved from the requested nodemask,
but perhaps starting from a different base.

Whether this is worth doing at all, or worth improving further, is
arguable: queue_folio_required() is right not to care about the precise
placement on interleaved nodes; but this little effort seems appropriate.

[hughd@google.com: do vma_iter search under mmap_write_unlock()]
  Link: https://lkml.kernel.org/r/3311d544-fb05-a7f1-1b74-16aa0f6cd4fe@google.com
Link: https://lkml.kernel.org/r/77954a5-9c9b-1c11-7d5c-3262c01b895f@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tejun heo <tj@kernel.org>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>