resize-fs: Put minimal ext4 size in the same ballpark as the other filesystems

resize-fs: Put minimal ext4 size in the same ballpark as the other filesystems

TEST-46-HOMED fails on ext4 because the filesystem is deemed to small
for activation by cryptsetup. Let's bump the minimal filesystem size for
ext4 a bit to be in the same ballpark as ext4 and btrfs to avoid weird
errors due to impossibly small filesystems.

Also use U64_MB while we're touching this.

(cherry picked from commit ae07feb401ff70b122650ac01041021703d4b8ad)