dev/dax: fix uninitialized variable build warning

Fix this build warning:

warning: 'phys' may be used uninitialized in this function
[-Wuninitialized]

As reported here:

https://lkml.org/lkml/2017/10/16/152
http://kisskb.ellerman.id.au/kisskb/buildresult/13181373/log/



Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>