[Merge] lp:~csurbhi/partman-btrfs/testing-subvol into lp:~ubuntu-core-dev/partman-btrfs/ubuntu

[Surbhi Palande]

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)
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https://code.launchpad.net/~csurbhi/partman-btrfs/testing-subvol/+merge/47975
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