ext2, ext4, xfs: retrieve dax_device for iomap operations

In preparation for converting fs/dax.c to use dax_direct_access()
instead of bdev_direct_access(), add the plumbing to retrieve the
dax_device associated with a given block_device.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

include/linux/iomap.h

@@ -41,6 +41,7 @@ struct iomap {
 	u16			type;	/* type of mapping */
 	u16			flags;	/* flags for mapping */
 	struct block_device	*bdev;	/* block device for I/O */
+	struct dax_device	*dax_dev; /* dax_dev for dax operations */
 };
 
 /*