dax: convert to the cdev api

A goal of the device-DAX interface is to be able to support many
exclusive allocations (partitions) of performance / feature
differentiated memory.  This count may exceed the default minors limit
of 256.

As a result of switching to an embedded cdev the inode-to-dax_dev
conversion is simplified, as well as reference counting which can switch
to the cdev kobject lifetime.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>