libsnapshot: update cow estimation

Cow estimator needs to update next_data_pos_ to be in the correct
position

Test: th
Change-Id: I1e3f2c9434573197e840be5637a90c679610ac4e

fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp

@@ -322,6 +322,7 @@ bool CowWriterV3::WriteOperation(const CowOperationV3& op, const void* data, siz
     if (IsEstimating()) {
         header_.op_count++;
         header_.op_count_max++;
+        next_data_pos_ += op.data_length;
         return true;
     }