hugetlb: remove uses of folio_mapcount_ptr

Use the entire_mapcount field directly.

Link: https://lkml.kernel.org/r/20230111142915.1001531-14-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>