Don't growfs if we're going to autosave anyway

[jlebon]

Currently, we do ignition-ostree-growfs and then in ignition-ostree-transposefs-save.service check if the agcount exceeds the threshold to know if to kick in the autosave feature. The problem is that growing the rootfs will cause any mounting of the rootfs to be extremely slow.

We should instead just have ignition-ostree-growfs not do the actual filesystem growing (but still do the partition growing) if it detects that it would meet the threshold and e.g. drop a file for ignition-ostree-transposefs-save.service to know that it needs to autosave (or have the latter figure it out on its own if that's easier).