volume_id: add support for btrfs

function                                             old     new   delta
volume_id_probe_btrfs                                  -      96     +96
fs2                                                   48      52      +4
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(add/remove: 1/0 grow/shrink: 1/0 up/down: 100/0)             Total: 100 bytes

Signed-off-by: Vladimir Dronnikov <dronnikov@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
6 files changed