sctp: delete the nested flexible array hmac

This patch deletes the flexible-array hmac[] from the structure
sctp_authhdr to avoid some sparse warnings:

  # make C=2 CF="-Wflexible-array-nested" M=./net/sctp/
  net/sctp/auth.c: note: in included file (through include/net/sctp/structs.h, include/net/sctp/sctp.h):
  ./include/linux/sctp.h:735:29: warning: nested flexible array
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sctp.h

@@ -727,7 +727,7 @@ struct sctp_addip_chunk {
 struct sctp_authhdr {
 	__be16 shkey_id;
 	__be16 hmac_id;
-	__u8   hmac[];
+	/* __u8   hmac[]; */
 };
 
 struct sctp_auth_chunk {

net/sctp/auth.c

@@ -738,7 +738,7 @@ void sctp_auth_calculate_hmac(const struct sctp_association *asoc,
 
 	tfm = asoc->ep->auth_hmacs[hmac_id];
 
-	digest = auth->auth_hdr.hmac;
+	digest = (u8 *)(&auth->auth_hdr + 1);
 	if (crypto_shash_setkey(tfm, &asoc_key->data[0], asoc_key->len))
 		goto free;
 

net/sctp/sm_statefuns.c

@@ -4391,7 +4391,7 @@ static enum sctp_ierror sctp_sf_authenticate(
 	 *  3. Compute the new digest
 	 *  4. Compare saved and new digests.
 	 */
-	digest = auth_hdr->hmac;
+	digest = (u8 *)(auth_hdr + 1);
 	skb_pull(chunk->skb, sig_len);
 
 	save_digest = kmemdup(digest, sig_len, GFP_ATOMIC);