Update reclaim disk space script

The script uses systemd-run, which does the job in background.
We should take the the time and wait for the job to finish.

Maybe some functional tests suffer from not really freed disk
space and fail because of this.

We also add some trimming in the end of the script.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #14554