nvme: wire up fixed buffer support for nvme passthrough

if io_uring sends passthrough command with IORING_URING_CMD_FIXED flag,
use the pre-registered buffer for IO (non-vectored variant). Pass the
buffer/length to io_uring and get the bvec iterator for the range. Next,
pass this bvec to block-layer and obtain a bio/request for subsequent
processing.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Link: https://lore.kernel.org/r/20220930062749.152261-13-anuj20.g@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>