nvme: Add namespace mgmt and attachment support.

[mffbsdw_gmail.com]

Summary

Add nscreate/nsdelete and nsattach/nsdetach to nvmecontrol
with support for same in nvme driver supporting both nvd and nda periphs.

Test Plan

Tested nscreate/nsattach partition, mount, write/read and
nsdetach/nsdelete on HGST 4500 and Samsung PM963 nvme drives supporting
namespace and attachement command sets. Ensure existing nvme drives not
supporting namespace (Intel 750) continue to work as expected.