nvme-pci: remove nvmeq->tags

There is no real need to have a pointer to the tagset in
struct nvme_queue, as we only need it in a single place, and that place
can derive the used tagset from the device and qid trivially.  This
fixes a problem with stale pointer exposure when tagsets are reset,
and also shrinks the nvme_queue structure.  It also matches what most
other transports have done since day 1.

Reported-by: Edmund Nadolski <edmund.nadolski@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>