iomap: add a page_done callback

This will be used by gfs2 to attach data to transactions for the journaled
data mode.  But the concept is generic enough that we might be able to
use it for other purposes like encryption/integrity post-processing in the
future.

Based on a patch from Andreas Gruenbacher.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>