api2/status: fix estimation bug

when a datastore has enough data to calculate the estimated full date,
but always has exactly the same usage, the factor b of the regression
is '0'

return 0 for that case so that the gui can show 'never' instead of
'not enough data'

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/api2/status.rs

@@ -161,6 +161,8 @@ fn datastore_status(
                         if b != 0.0 {
                             let estimate = (1.0 - a) / b;
                             entry["estimated-full-date"] = Value::from(estimate.floor() as u64);
+                        } else {
+                            entry["estimated-full-date"] = Value::from(0);
                         }
                     }
                 }