mm: reimplement folio_is_device_private()

For callers of folio_is_device_private(), we save a folio->page->folio
conversion.  Callers of is_device_private_page() simply move the
conversion of page->folio from the implementation of page_pgmap() to
is_device_private_page().

Link: https://lkml.kernel.org/r/20250805172307.1302730-9-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>