storage: Creating btrfs subvolumes doesn't check for duplicate mount points

[mvollmer]

Cockpit is supposed to prevent people from assigning the same mount point to multiple things. This doesn't work within the subvolumes of a single btrfs filesystem however.

• Create btrfs filesystem, mount at "/run/butter"
• Create subvolume in it, also mount at "/run/butter"
  => Cockpit writes two fstab entries for "/run/butter"

Instead it should prevent the creation of the subvolume with the error "Mount point /run/butter already used for top-level subvolume of ..."