Fix multiple bugs in dst match:

- ".me = THIS_MODULE" is missing
- The cast of the pointer to the next IPv6 extension header is wrong.
- hdrlen may underflow.
- (u16)*optdesc causes to alignment problem.
- The calculation of the offset to next option is wrong. In the case
  that the type isn't 0, it should be "Opt Data Len" field + 2
  (see RFC2460).