mm: add pmd_folio()

Convert directly from a pmd to a folio without going through another
representation first.  For now this is just a slightly shorter way to
write it, but it might end up being more efficient later.

Link: https://lkml.kernel.org/r/20240326202833.523759-4-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>