mm/memfd: refactor memfd_tag_pins() and memfd_wait_for_pins()

Patch series "mm: remove total_mapcount()", v2.

Let's remove the remaining user from mm/memfd.c so we can get rid of
total_mapcount().


This patch (of 2):

Both functions are the remaining users of total_mapcount().  Let's get rid
of the calls by converting the code to folios.

As it turns out, the code is unnecessarily complicated, especially:

1) We can query the number of pagecache references for a folio simply via
   folio_nr_pages(). This will handle other folio sizes in the future
   correctly.

2) The xas_set(xas, page->index + cache_count) call to increment the
   iterator for large folios is not required. Remove it.

Further, simplify the XA_CHECK_SCHED check, counting each entry exactly
once.

Memfd pages can be swapped out when using shmem; leave xa_is_value()
checks in place.

Link: https://lkml.kernel.org/r/20240226141324.278526-1-david@redhat.com
Link: https://lkml.kernel.org/r/20240226141324.278526-2-david@redhat.com
Co-developed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>