Revert "nvme-pci: remove last_sq_tail"

Revert "nvme-pci: remove last_sq_tail"

[ Upstream commit 38210800bf66d7302da1bb5b624ad68638da1562 ]

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 54b2fcee1db041a83b52b51752dade6090cf952f.

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>