mm/rmap: use folio->_mapcount for small folios

We have some cases left whereby we operate on small folios and still refer
to page->_mapcount.  Let's just use folio->_mapcount instead, which
currently still overlays page->_mapcount, so no change.

This change will make it easier to later spot any remaining users of
page->_mapcount that target tail pages.

Link: https://lkml.kernel.org/r/20240816103246.719209-1-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>