iov_iter: Convert iter_xarray to use folios

Take advantage of how kmap_local_folio() works to simplify the loop.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>