nvme-pci: dma read memory barrier for completions

Control dependencies do not guarantee load order across the condition,
allowing a CPU to predict and speculate memory reads.

Commit 324b494c

 inlined verifying a new completion with its
handling. At least one architecture was observed to access the contents
out of order, resulting in the driver using stale data for the
completion.

Add a dma read barrier before reading the completion queue entry and
after the condition its contents depend on to ensure the read order is
determinsitic.

Reported-by: John Garry <john.garry@huawei.com>
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Tested-by: John Garry <john.garry@huawei.com>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>