Release onexit/events with any missed zfsdev_state

Release onexit/events with any missed zfsdev_state

Linux and FreeBSD will most likely never see this issue.
On macOS when kext is unloaded, but zed is still connected, zed
will be issued ENODEV. As the cdevsw is released, the kernel
will not have zfsdev_release() called to release minor/onexit/events,
and it "leaks". This ensures it is cleaned up before unload.

Changed the for loop from zsprev, to zsnext style, for less
code duplication.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #10700