Revert "nvme-pci: remove last_sq_tail"

Multiple CPUs may be mapped to the same hctx, allowing mulitple
submission contexts to attempt commit_rqs(). We need to verify we're
not writing the same doorbell value multiple times since that's a spec
violation.

Revert commit 54b2fcee.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1878596


Reported-by: "B.L. Jones" <brandon.gustav@googlemail.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>