fix #6553: lvmthin: implement volume_rollback_is_possible sub

Because LvmThinPlugin.pm uses LVMPlugin.pm as a base, it inherits the
`volume_rollback_is_possible()` subroutine added in eda88c94. Its
implementation however causes snapshot rollbacks to fail with
"can't rollback snapshot for 'raw' volume".

Fix this by implementing `volume_rollback_is_possible()`.

Closes: #6553
Signed-off-by: Max R. Carrara <m.carrara@proxmox.com>

src/PVE/Storage/LvmThinPlugin.pm

@@ -363,6 +363,12 @@ sub volume_snapshot {
     # disabling autoactivation not needed, as -s defaults to --setautoactivationskip y
 }
 
+sub volume_rollback_is_possible {
+    my ($class, $scfg, $storeid, $volname, $snap, $blockers) = @_;
+
+    return 1;
+}
+
 sub volume_snapshot_rollback {
     my ($class, $scfg, $storeid, $volname, $snap) = @_;