iomap: Rename page_prepare handler to get_folio

The ->page_prepare() handler in struct iomap_page_ops is now somewhat
misnamed, so rename it to ->get_folio().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>