dccp: Always generate a Reset in response to option errors

RFC4340 states that if a packet is received with an option error (such as a
Mandatory Option as the last byte of the option list), the endpoint should
repond with a Reset.

In the LISTEN and RESPOND states, the endpoint correctly reponds with Reset,
while in the REQUEST/OPEN states, packets with option errors are just ignored.

The packet sequence is as follows:

Case 1:

  Endpoint A                           Endpoint B
  (CLOSED)                             (CLOSED)

               <----------------       REQUEST

  RESPONSE     ----------------->      (*1)
  (with invalid option)
               <----------------       RESET
                                       (with Reset Code 5, "Option Error")

  (*1) currently just ignored, no Reset is sent

Case 2:

  Endpoint A                           Endpoint B
  (OPEN)                               (OPEN)

  DATA-ACK     ----------------->      (*2)
  (with invalid option)
               <----------------       RESET
                                       (with Reset Code 5, "Option Error")

  (*2) currently just ignored, no Reset is sent

This patch fixes the problem, by generating a Reset instead of silently
ignoring option errors.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>