libnvdimm, namespace: fix label initialization to use valid seq numbers

The set of valid sequence numbers is {1,2,3}. The specification
indicates that an implementation should consider 0 a sign of a critical
error:

    UEFI 2.7: 13.19 NVDIMM Label Protocol

    Software never writes the sequence number 00, so a correctly
    check-summed Index Block with this sequence number probably indicates a
    critical error. When software discovers this case it treats it as an
    invalid Index Block indication.

While the expectation is that the invalid block is just thrown away, the
Robustness Principle says we should fix this to make both sequence
numbers valid.

Fixes: f524bf27

 ("libnvdimm: write pmem label set")
Cc: <stable@vger.kernel.org>
Reported-by: Juston Li <juston.li@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>