t/io_uring: Vectored fixed buffer test support for nvme passthrough path

t/io_uring: Vectored fixed buffer test support for nvme passthrough path

The current kernel NVMe passthrough path already supports vectored IO
when using fixed buffers, but fio has not yet adapted it. This patch
aims to add a corresponding test interface in fio.

Test results:

taskset -c 1 t/io_uring -b512 -d64 -c2 -s2 -p1 -F1 -B1 -O0 -n1 -V1 -u1 -r4 /dev/ng1n1
submitter=0, tid=6179, file=/dev/ng1n1, nfiles=1, node=-1
polled=1, fixedbufs=1, register_files=1, buffered=1, QD=64
Engine=io_uring, sq_ring=64, cq_ring=64
IOPS=289.78K, BW=141MiB/s, IOS/call=1/1
IOPS=294.68K, BW=143MiB/s, IOS/call=1/1
IOPS=295.26K, BW=144MiB/s, IOS/call=1/1
Exiting on timeout
Maximum IOPS=295.26K

taskset -c 1 t/io_uring -b512 -d64 -c2 -s2 -p1 -F1 -B1 -O0 -n1 -V0 -u1 -r4 /dev/ng1n1
submitter=0, tid=6183, file=/dev/ng1n1, nfiles=1, node=-1
polled=1, fixedbufs=1, register_files=1, buffered=1, QD=64
Engine=io_uring, sq_ring=64, cq_ring=64
IOPS=292.31K, BW=142MiB/s, IOS/call=1/1
IOPS=295.79K, BW=144MiB/s, IOS/call=1/1
IOPS=290.78K, BW=141MiB/s, IOS/call=1/1
Exiting on timeout
Maximum IOPS=295.79K

Signed-off-by: Xiaobing Li <xiaobing.li@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>