os/bluestore: adds assert for device mounted before expand

Signed-off-by: Joshua Blanch <joshua.blanch@clyso.com>

src/os/bluestore/BlueStore.cc

@@ -9267,6 +9267,7 @@ int BlueStore::expand_devices(ostream& out)
                 << std::dec << "(" << byte_u_t(size) << ")"
                 << std::endl;
           }
+          ceph_assert(mounted);
           bluefs->expand_device(devid, size, old_size);
         }
       }