Debug cv_destroy() with mutex held

Debug cv_destroy() with mutex held

There still appears to be a race in the condition variables where
->cv_mutex is set after we are woken from the cv_destroy wait queue.
This might be possible when cv_destroy() is called immediately after
cv_broadcast().  We had some troubles with this previously but
there may still be a small race, see commit d599e4f.

The following patch closes one small race and improves the ASSERTs
such that they log the offending value.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
zfsonlinux/zfs#943