nvme: Rename min_page_size to page_size and save mps

The Memory Page Size sets the basic unit of operation for the drive. We
currently set this to the drive's minimum page size, but we could set it
to any page size the drive supports in the future. Replace min_page_size
(it's now unused for that purpose) with page_size to reflect this and
cache the MPS we want to use. Use NVME_MPS_SHIFT to compute page_size.

Sponsored by: Netflix
Reviewed by: chuck
Differential Revision: https://reviews.freebsd.org/D34868