libnvdimm, dimm: fix dpa reservation vs uninitialized label area

At initialization time the 'dimm' driver caches a copy of the memory
device's label area and reserves address space for each of the
namespaces defined.

However, as can be seen below, the reservation occurs even when the
index blocks are invalid:

 nvdimm nmem0: nvdimm_init_config_data: len: 131072 rc: 0
 nvdimm nmem0: config data size: 131072
 nvdimm nmem0: __nd_label_validate: nsindex0 labelsize 1 invalid
 nvdimm nmem0: __nd_label_validate: nsindex1 labelsize 1 invalid
 nvdimm nmem0: : pmem-6025e505: 0x1000000000 @ 0xf50000000 reserve <-- bad

Gate dpa reservation on the presence of valid index blocks.

Cc: <stable@vger.kernel.org>
Fixes: 4a826c83 ("libnvdimm: namespace indices: read and validate")
Reported-by: Krzysztof Rusocki <krzysztof.rusocki@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>