extend-filesystems: re-add support for extending ext4 filesystems

One important historical note: this differs from our ext4 resize
implementation that predated btrfs:
 - Not bothering with C, this bash script has been working well enough.
 - Previously ext4 was resized off-line, on-line avoids a long pause
   during boot and is now significantly faster that it used to be.

In some quick qemu based tests on-line resizes with small changes (2GB
to 7GB) took a few seconds while large changes (6GB to 5TB) took about a
minute. For comparison an off-line resize to 5TB took approximately
forever. (I gave up and killed the VM)