Add Host Memory Buffer support to nvme(4).

This allows cheapest DRAM-less NVMe SSDs to use some of host RAM (about
1MB per 1GB on the devices I have) for its metadata cache, significantly
improving random I/O performance. Device reports minimal and preferable
size of the buffer. The code limits it to 1% of physical RAM by default.
If the buffer can not be allocated or below minimal size, the device will
just have to work without it.

MFC after: 2 weeks
Relnotes: yes
Sponsored by: iXsystems, Inc.