btrfs mode: check root subvolume if default subvolume is not the root subvoume

[second2050]

Is your feature request related to a problem? Please describe.
this might be related to issue #90 as timeshift currently doesn't recognize the @ subvolume if the default subvolume is not subvolid=5/ID 5 (FS_TREE)

Describe the solution you'd like
If the @ subvolume isn't found on the current default subvolume timeshift should search for it on the root subvolume.
And also in that case save the snapshots on the root subvolume.

Or for a more powerful solution: let the user choose the subvolume that is to be backed up themselves.

Describe alternatives you've considered
My current workaround/solution is to have the default subvolume point to the root subvolume, were in my case @ and @home are located, which in my opinion

Additional context
"root subvolume" here refers here to the btrfs subvolume with the ID 5

[mareksamec]

I want to chip in here because as @second2050 mentioned in the issue #90, you need to modify the kernel parameters to make it work, which is not great.

Letting the user choose the subvolume is the way to go. It's been requested in the original repo in other issues many times.