iov_iter: convert iter_xarray_populate_pages() to use folios

ITER_XARRAY is exclusively used with xarrays that contain folios, not
pages, so extract folio pointers from it, not page pointers.  Removes a
hidden call to compound_head() and a use of find_subpage().

Link: https://lkml.kernel.org/r/20250402210612.2444135-4-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>