filesystem-dax: convert to dax_direct_access()

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: Christoph Hellwig <[email protected]>
Signed-off-by: Dan Williams <[email protected]>

Author: djbw