sctp: correct bounds check in sctp_setsockopt_auth_key

The bonds check to prevent buffer overlflow was not exactly
right.  It still allowed overflow of up to 8 bytes which is
sizeof(struct sctp_authkey).

Since optlen is already checked against the size of that struct,
we are guaranteed not to cause interger overflow either.
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -3144,7 +3144,7 @@ static int sctp_setsockopt_auth_key(struct sock *sk,
 		goto out;
 	}
 
-	if (authkey->sca_keylength > optlen) {
+	if (authkey->sca_keylength > optlen - sizeof(struct sctp_authkey)) {
 		ret = -EINVAL;
 		goto out;
 	}