iomap: switch readahead and readpage to use iomap_iter

Switch the page cache read functions to use iomap_iter instead of
iomap_apply.

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