zio_ioctl() is the only user of zio_flush(), and its structure and flag
use is fairly specific to flushing. So here we bring the guts of
zio_ioctl() into zio_flush(), allowing some light reorganising (mostly
around how zio_nowait() is called) and a better signature.
This will help in the future when changing the way flush works, as its
clear where the change should be made and no wondering if zio_ioctl() is
being used somewhere else.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
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