nvme: new define for size of host memory buffer sizes

The nvme spec defines the various fields that specify sizes for host
memory buffers in terms of 4096 chunks. So, rather than use a bare 4096
here, use NVME_HMB_UNITS. This is explicitly not the host page size of
4096, nor the default memory page size (mps) of the NVMe drive, but its
own thing and needs its own define.

No functional change is intended, only the logical spelling of 4k.

Sponsored by: Netflix