pmem: don't allocate unused major device number

When alloc_disk(0) or alloc_disk-node(0, XX) is used, the ->major
number is completely ignored:  all devices are allocated with a
major of BLOCK_EXT_MAJOR.

So there is no point allocating pmem_major.

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