nvdimm/blk: don't allocate unused major device number

When alloc_disk(0) is used ->major is completely ignored, all devices
are allocated with a "major" of BLOCK_EXT_MAJOR.

So don't allocate nd_blk_major

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