Add more checks.

Add more checks.

Check that the payload specified in a Jumbo Payload option isn't smaller
than the total number of bytes worth of extension headers; if it is,
report truncation.

Check that:

1) we don't have more than one Jumbo Payload option;
2) we don't have a Jumbo Payload option if the payload length in the
IPv6 header was non-zero;
3) we don't have a Jumbo Payload option with a value < 65536.