libnvdimm/namsepace: Don't set claim_class on error

libnvdimm/namsepace: Don't set claim_class on error

Don't leave claim_class set to an invalid value if an error occurs in
btt_claim_class().

While we are here change the return type of __holder_class_store() to be
clear about the values it is returning.

This was found via code inspection.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20190925211348.14082-1-ira.weiny@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>