Harddisk: Collect "-O" options together for mkfs.ext4

Apparently, mkfs.ext4 doesn't parse multiple "-O" options any more.
So collect them into a single set and pass them as a comma list.

This fixes this error message when you initialize a big disk:
mke2fs 1.42.8 (20-Jun-2013)
mkfs.ext4: specifying a cluster size requires the bigalloc feature