The default root subvol comes with its own limitations and it might be
better off changing the default subvol upon creation. This should also
let us create hidden subvols that may be used for storing snapshots,
without exposing them to the end-user.
PVCs are data volumes most of the times, and reserving space for system
tasks is probably unnecessary.
The user can still modify a specific PVC's reserved blocks through the
`tune2fs` command.
Summary: So that it's possible to use it with any other blockdevice provider.
Test Plan: N/A
Reviewers: sina_rad, h.marvi, mhyousefi, s.afshari
Differential Revision: https://phab.hamravesh.ir/D870
