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>

include/linux/iomap.h

@@ -57,6 +57,7 @@ struct iomap {
 	struct block_device	*bdev;	/* block device for I/O */
 	struct dax_device	*dax_dev; /* dax_dev for dax operations */
 	void			*inline_data;
+	void			*private; /* filesystem private */
 
 	/*
 	 * Called when finished processing a page in the mapping returned in