and therefore we need a way for ioctl handlers to run in that thread
in GEOM. Rather than invent a complicated registration system to
recognize which ioctl handler to use for a given ioctl, we still
schedule all ioctls down the tree as bio transactions but add a
special return code that means "call me directly" and have the
geom_dev layer do that.
Use this for all ioctls that make it as far as a diskdriver to
avoid any backwards compatibility problems.
Requested by: scottl
Sponsored by: DARPA & NAI Labs
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