device-dax: Add /sys/class/dax backwards compatibility

On the expectation that some environments may not upgrade libdaxctl
(userspace component that depends on the /sys/class/dax hierarchy),
provide a default / legacy dax_pmem_compat driver. The dax_pmem_compat
driver implements the original /sys/class/dax sysfs layout rather than
/sys/bus/dax. When userspace is upgraded it can blacklist this module
and switch to the dax_pmem driver going forward.

CONFIG_DEV_DAX_PMEM_COMPAT and supporting code will be deleted according
to the dax_pmem entry in Documentation/ABI/obsolete/.

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

Documentation/ABI/obsolete/sysfs-class-dax

@@ -0,0 +1,22 @@
+What:           /sys/class/dax/
+Date:           May, 2016
+KernelVersion:  v4.7
+Contact:        linux-nvdimm@lists.01.org
+Description:	Device DAX is the device-centric analogue of Filesystem
+		DAX (CONFIG_FS_DAX).  It allows memory ranges to be
+		allocated and mapped without need of an intervening file
+		system.  Device DAX is strict, precise and predictable.
+		Specifically this interface:
+
+		1/ Guarantees fault granularity with respect to a given
+		page size (pte, pmd, or pud) set at configuration time.
+
+		2/ Enforces deterministic behavior by being strict about
+		what fault scenarios are supported.
+
+		The /sys/class/dax/ interface enumerates all the
+		device-dax instances in the system. The ABI is
+		deprecated and will be removed after 2020. It is
+		replaced with the DAX bus interface /sys/bus/dax/ where
+		device-dax instances can be found under
+		/sys/bus/dax/devices/