[DCCP] options: Fix some aspects of mandatory option processing

According to dccp draft (draft-ietf-dccp-spec-13.txt) section 5.8.2
(Mandatory Option) the following patch correct the handling of the
following cases:

1) "... and any Mandatory options received on DCCP-Data packets MUST be
  ignored."

2) "The connection is in error and should be reset with Reset Code 5, ...
  if option O is absent (Mandatory was the last byte of the option list), or
  if option O equals Mandatory."

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: David S. Miller <davem@davemloft.net>