nvme-pci: add support for sgl metadata

[ Upstream commit 979c6342 ]

Supporting this mode allows creating and merging multi-segment metadata
requests that wouldn't be possible otherwise. It also allows directly
using user space requests that straddle physically discontiguous pages.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Stable-dep-of: 00817f0f ("nvme-ioctl: fix leaked requests on mapping error")
Signed-off-by: Sasha Levin <sashal@kernel.org>