Autotrim High Load Average Fix

Switch from cv_wait() to cv_wait_idle() in vdev_autotrim_wait_kick(),
which should mitigate the high load average while waiting.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: jxdking <lostking2008@hotmail.com>
Closes #15781