fio: add hipri option to sg engine

fio: add hipri option to sg engine

Adds hipri option to the Linux sg driver engine. This turns on the
SGV4_FLAG_HIPRI flag in recent sg drivers (January 2021) on READ
and WRITE commands (and not on UNMAP (trim), VERIFY, etc). Uses
blk_poll() and the mq_poll() callback in SCSI LLDs. The mechanism
is also called "iopoll".

The Linux sg engine in fio uses the struct sg_io_hdr based interface
known as the sg driver "v3" interface.
Linux sg drivers in the kernel prior to January 2021 (sg version
4.0.12) will just ignore the SGV4_FLAG_HIPRI flag and do normal
completions where LLDs indicate command completion with a (software)
interrupt or similar mechanism.

Update fio.1 (manpage) with new hipri sg engine option.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>