Allow btrfs subvolumes without mountpoint

[achilleas-k]

Btrfs subvolumes don't need to be mounted. They are always accessible at the location where they are created. This is quite common for nested subvolumes (beyond the top-level ones). For example, consider the following subvolume layout, as it may appear on the btrfs volume itself:

/
|- +subvolumes
   |- root
   |- var
   |  |- log
   |- home

In this case, the following mountpoints would be specified in /etc/fstab (or the mount units) for our images:

UUID=<btrfs volume uuid>    /                     btrfs     ...,subvol=/+subvolumes/root     0 0
UUID=<btrfs volume uuid>    /var                  btrfs     ...,subvol=/+subvolumes/var      0 0
UUID=<btrfs volume uuid>    /var/log              btrfs     ...,subvol=/+subvolumes/var/log  0 0
UUID=<btrfs volume uuid>    /home                 btrfs     ...,subvol=/+subvolumes/home     0 0

The third line for /var/log is unnecessary [1].

Making the mountpoint an optional property also allows users to simplify their mount structure by creating only a single subvolume with a mountpoint and then structuring their subvolumes under that:

/
|- +subvolumes
   |- root
      |- var
      |  |- log
      |- home

requiring only

UUID=<btrfs volume uuid>    /                     btrfs     ...,subvol=/+subvolumes/root     0 0

(or no fstab at all).

[1] Mount options might be a reason to have a subvolume mounted at its original path, however this doesn't work for most options currently:

Most mount options apply to the whole filesystem and only options in the first mounted subvolume will take effect. This is due to lack of implementation and may change in the future. This means that (for example) you can’t set per-subvolume nodatacow, nodatasum, or compress using mount options. This should eventually be fixed, but it has proved to be difficult to implement correctly within the Linux VFS framework.

https://btrfs.readthedocs.io/en/latest/btrfs-man5.html#btrfs-specific-mount-options