nvmet: implement endurance groups

Most of the returned information is just stubbed data. The target must
support these in order to report rotational media. Since this driver
doesn't know any better, each namespace is its own endurance group with
the engid value matching the nsid.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>