nvme/nvmf: Add NVME_GET_CONTROLLER_DATA ioctl to fetch cached cdata

Both nvme and nvmf cache a copy of the controller's identify data in
the softc. Add an ioctl to fetch this copy of the cdata. This is
primarily useful for allowing commands like 'nvmecontrol devlist' to
work against a disconnected Fabrics host.

Reviewed by: dab, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D48218