mm: introduce page_mapcount_is_type()

Resolve the awkward "and add one to this opaque constant" test into a
self-documenting inline function.

Link: https://lkml.kernel.org/r/20240821173914.2270383-3-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>