brd: add dax_operations support

    
Setup a dax_inode to have the same lifetime as the brd block device and
add a ->direct_access() method that is equivalent to
brd_direct_access(). Once fs/dax.c has been converted to use
dax_operations the old brd_direct_access() will be removed.

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