nvme: enable vectored registered bufs for passthrough cmds

nvme already supports registered buffers for non-vectored io_uring
passthrough commands, enable it for the vectored mode as well. It takes
an iovec, each entry of which should contain a range within the same
registered buffer specificied in sqe->buf_index.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>