nvme: Further refinements in Host Memory Buffer Sizing

Host Memory Buffer units are a mix. For those in the identify structure,
the size is in 4kiB chunks. For specifying the buffer description,
though, they are in terms of the drive's MPS. Add comments to this
effect and change PAGE_SIZE to ctrlr->page_size where needed, as well as
correct a mistaken use of NVME_HPS_UNITS in 214df80a9cb3 as pointed out
by rpokala@ after the commit. No functional change is intended, as
page_size is still 4k which matches all current hosts' PAGE_SIZE, but to
support 16k pages on arm, we need to differentiate these two cases.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D34871