Merge pull request #698 from giuseppe/counter-check-for-unmount

counter: check for external umounts

drivers/counter.go

@@ -51,6 +51,10 @@ func (c *RefCounter) incdec(path string, infoOp func(minfo *minfo)) int {
 		if c.checker.IsMounted(path) {
 			m.count++
 		}
+	} else if !c.checker.IsMounted(path) {
+		// if the unmount was performed outside of this process (e.g. conmon cleanup)
+		//the ref counter would lose track of it.  Check if it is still mounted.
+		m.count = 0
 	}
 	infoOp(m)
 	count := m.count