nvme: add vectored-io support for uring-cmd

nvme: add vectored-io support for uring-cmd

wire up support for async passthru that takes an array of buffers (using
iovec). Exposed via a new op NVME_URING_CMD_IO_VEC. Same 'struct
nvme_uring_cmd' is to be used with -

1. cmd.addr as base address of user iovec array
2. cmd.data_len as count of iovec array elements

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220511054750.20432-6-joshi.k@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>