set vm.laptop_mode to 5 seconds via sysctl

kernel docs for laptop_mode: when enabling this,

any physical disk I/O (that might have caused the hard disk to spin up)
causes Linux to flush all dirty blocks. The result of this is that after
a disk has spun down, it will not be spun up anymore to write dirty blocks,
because those blocks had already been written immediately after the most
recent read operation. The value of the laptop_mode knob determines the
time between the occurrence of disk I/O and when the flush is triggered.
A sensible value for the knob is 5 seconds. Setting the knob to 0 disables
laptop mode.
5 jobs for vm_laptop_mode in 4 minutes and 40 seconds (queued for 2 seconds)