Now that a dax_device is plumbed through all dax-capable drivers we can
switch from block_device_operations to dax_operations for invoking
->direct_access.

This also lets us kill off some usages of struct blk_dax_ctl on the way
to its eventual removal.
Suggested-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>