Online discard asks the disk to erase flash memory cells as soon
as files are deleted. This gives the disk more freedom to choose
where to place new files, so it improves performance.
On older kernel versions, and on really bad disks, this can reduce
performance so we add an option to disable it.
Since fstrim is pointless when online discard is enabled, we
don't configure it if online discard is selected.
I tested it on an AWS i3.large instance, the flag showd up in
`mount` after configuration.
Closes#9608
a19d00ef9b