iomap: pass an iomap_iter to various buffered I/O helpers

Pass the iomap_iter structure instead of individual parameters to
various internal helpers for buffered I/O.

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>