libnvdimm, dax: reserve space to store labels for device-dax

We may want to subdivide a device-dax range into multiple devices so
that each can have separate permissions or naming.  Reserve 128K of
label space by default so we have the capability of making allocation
decisions persistent.  This reservation is not something we can add
later since it would result in the default size of a device-dax range
changing between kernel versions.

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