[SCTP]: Fix kernel panic while received AUTH chunk while enabled auth

If STCP is started while /proc/sys/net/sctp/auth_enable is set 0 and
association is established between endpoints. Then if
/proc/sys/net/sctp/auth_enable is set 1, a received AUTH chunk will
cause kernel panic.

Test as following:
step 1: echo 0> /proc/sys/net/sctp/auth_enable
step 2:

   SCTP client                  SCTP server
      INIT          --------->
                    <---------   INIT-ACK
      COOKIE-ECHO   --------->
                    <---------   COOKIE-ACK
step 3:
    echo 1> /proc/sys/net/sctp/auth_enable
step 4:
   SCTP client                  SCTP server
       AUTH        ----------->  Kernel Panic


This patch fix this probleam to treat AUTH chunk as unknow chunk if peer 
has initialized with no auth capable.

> Sorry for the delay.  Was on vacation without net access.
>
> Wei Yongjun wrote:
>>
>>
>> This patch fix this probleam to treat AUTH chunk as unknow chunk if 
>> peer has initialized with no auth capable.
>>
>> Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
>
> Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
>
>>
Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/sm_statefuns.c

@@ -3865,6 +3865,10 @@ sctp_disposition_t sctp_sf_eat_auth(const struct sctp_endpoint *ep,
 	struct sctp_chunk *err_chunk;
 	sctp_ierror_t error;
 
+	/* Make sure that the peer has AUTH capable */
+	if (!asoc->peer.auth_capable)
+		return sctp_sf_unk_chunk(ep, asoc, type, arg, commands);
+
 	if (!sctp_vtag_verify(chunk, asoc)) {
 		sctp_add_cmd_sf(commands, SCTP_CMD_REPORT_BAD_TAG,
 				SCTP_NULL());