ZTS: Fix zfs_mount.kshlib cleanup

Update cleanup_filesystem to use destroy_dataset when performing
cleanup. This ensures the destroy is retried if the pool is busy
preventing occasional failures.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10358