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

alloc_disk(0) does not require or use a ->major number,
all devices are allocated with a major of BLOCK_EXT_MAJOR.

So don't allocate btt_major.

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