mm: reimplement folio_is_fsdax()

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

Link: https://lkml.kernel.org/r/20250805172307.1302730-11-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>