mkfs's arguments are
mkfs [options] [-t type] [fs-options] device [size]
So it seems our MKFS_OPTS are really supposed to be fs-options, rather
than options to mkfs itself.
Why didn't we notice? It's quite a trap -- mkfs.ext2 has a "-t"
option, so when we're calling
$ mkfs -i 4096 ... -t ext4 ...
We actually just fall-back to the default from the mkfs wrapper which
is mkfs.ext2 which works! But when you make that, say, xfs, we're not
calling the right wrapper at all.
Also update documentation
Closes-Bug: #1648287
Change-Id: I3ea5807088ab361bd9c235c07fb1553fbaf9178b
5e2330d89c