Code review comment for lp:~csurbhi/partman-btrfs/testing-subvol

Following change added:
Need to remove the subvol=@ before creating the subvolume from the mount options. This removal could be when options="defaults,subvol=@" or options="subvol=@". Added a clause to remove the "subvol=@" when no comma is present. (Comments added in the changed code)