engines/devdax: Make detection of device-dax instances more robust

engines/devdax: Make detection of device-dax instances more robust

In preparation for the kernel switching device-dax instances from the
"/sys/class/dax" subsystem to "/sys/bus/dax" [1], teach the device-dax
instance detection to be subsystem-type agnostic.

Note that the subsystem switch will require an administrator, or distro
opt-in. The opt-in will either be at kernel compile time by disabling
the default compatibility driver in the kernel, or at runtime with a
modprobe policy to override which kernel module service device-dax
devices. The daxctl utility [2] will ship a command to install the
modprobe policy and include a man page that lists the potential
regression risk to older FIO and other userspace tools that are hard
coded to "/sys/class/dax".

[1]: https://lwn.net/Articles/770128/
[2]: https://github.com/pmem/ndctl/tree/master/daxctl

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>