Propagate bcachefs_filesystem mountOptions to subvolumes

[VlaDexa]

Make subvolume mountOptions also include options for the whole
filesystem, but only when the filesystems mountpoint is not specified

My reasoning behind this is that if user specifies mountOptions for the filesystem, but doesn't mount it, it's likely they want those options to apply to subvolumes of that filesystem.

[nothingnesses]

It might be worth either modifying the mounted_subvolumes_in_multi example in https://github.com/nix-community/disko/blob/master/example/bcachefs.nix (which doesn't have a mountpoint, so it should be suitable for this) or adding a new entry in bcachefs_filesystems, to add one or more mountOptions that will be applied to all its subvolumes, as well as updating or adding the corresponding test in https://github.com/nix-community/disko/blob/master/tests/bcachefs.nix , to demonstrate this works properly.

Alternatively, it might instead be worth adding an explicit subvolumesMountOptions configuration option for the bcachefs_filesystem type that can be used to apply the mount options listed to all its subvolumes, which I think would be less confusing. It'd be good to have corresponding examples and tests for this too.

Alternatively, you can just set the mountOptions for each of the subvolumes manually, which would require no code changes.