agent: fix wrong return value of set_sandbox_storage()

Function set_sandbox_storage() is designed to return true when the
reference count drops from 1 to 0. But current implementation always
return true no matter the reference count is, which may cause removing
an in use mountpoint.

Fixes: #88

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

src/agent/src/sandbox.rs

@@ -103,8 +103,9 @@ impl Sandbox {
                 *count -= 1;
                 if *count < 1 {
                     self.storages.remove(path);
+                    return true;
                 }
-                return true;
+                false
             }
         }
     }