mm: add folio_estimated_sharers()

Patch series "Convert various mempolicy.c functions to use folios", v4.

This patch series converts migrate_page_add() and queue_pages_required()
to migrate_folio_add() and queue_page_required().  It also converts the
callers of the functions to use folios as well, and introduces a helper
function to estimate the number of sharers of a folio.


This patch (of 6):

folio_estimated_sharers() takes in a folio and returns the precise number
of times the first subpage of the folio is mapped.

This function aims to provide an estimate for the number of sharers of a
folio.  This is necessary for folio conversions where we care about the
number of processes that share a folio, but don't necessarily want to
check every single page within that folio.

This is in contrast to folio_mapcount() which calculates the total number
of the times a folio and all its subpages are mapped.

Link: https://lkml.kernel.org/r/20230130201833.27042-1-vishal.moola@gmail.com
Link: https://lkml.kernel.org/r/20230130201833.27042-2-vishal.moola@gmail.com


Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Jane Chu <jane.chu@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>