iomap: add private pointer to struct iomap

Add a private pointer to struct iomap to allow filesystems to pass data
from iomap_begin to iomap_end.  Will be used by gfs2 for passing on the
on-disk inode buffer head.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
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>