nvme-pci: implement the HMB entry number and size limitations

Adds support for the new Host Memory Buffer Minimum Descriptor Entry Size
and Host Memory Maximum Descriptors Entries field that were added in
TP 4002 HMB Enhancements.  These allow the controller to advertise
limits for the usual number of segments in the host memory buffer, as
well as a minimum usable per-segment size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>