mm: use entire_mapcount in __page_dup_rmap()

Remove the use of the compound_mapcount_ptr() wrapper, and add an
assertion that we're not passing a tail page if we're duplicating a PMD.

Link: https://lkml.kernel.org/r/20230111142915.1001531-12-willy@infradead.org


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