sctp: delete the nested flexible array skip

This patch deletes the flexible-array skip[] from the structure
sctp_ifwdtsn/fwdtsn_hdr to avoid some sparse warnings:

  # make C=2 CF="-Wflexible-array-nested" M=./net/sctp/
  net/sctp/stream_interleave.c: note: in included file (through include/net/sctp/structs.h, include/net/sctp/sctp.h):
  ./include/linux/sctp.h:611:32: warning: nested flexible array
  ./include/linux/sctp.h:628:33: 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

@@ -603,7 +603,7 @@ struct sctp_fwdtsn_skip {
 
 struct sctp_fwdtsn_hdr {
 	__be32 new_cum_tsn;
-	struct sctp_fwdtsn_skip skip[];
+	/* struct sctp_fwdtsn_skip skip[]; */
 };
 
 struct sctp_fwdtsn_chunk {
@@ -620,7 +620,7 @@ struct sctp_ifwdtsn_skip {
 
 struct sctp_ifwdtsn_hdr {
 	__be32 new_cum_tsn;
-	struct sctp_ifwdtsn_skip skip[];
+	/* struct sctp_ifwdtsn_skip skip[]; */
 };
 
 struct sctp_ifwdtsn_chunk {

include/net/sctp/sctp.h

@@ -452,8 +452,8 @@ for (err = (struct sctp_errhdr *)((void *)chunk_hdr + \
 _sctp_walk_fwdtsn((pos), (chunk), ntohs((chunk)->chunk_hdr->length) - sizeof(struct sctp_fwdtsn_chunk))
 
 #define _sctp_walk_fwdtsn(pos, chunk, end)\
-for (pos = chunk->subh.fwdtsn_hdr->skip;\
-     (void *)pos <= (void *)chunk->subh.fwdtsn_hdr->skip + end - sizeof(struct sctp_fwdtsn_skip);\
+for (pos = (void *)(chunk->subh.fwdtsn_hdr + 1);\
+     (void *)pos <= (void *)(chunk->subh.fwdtsn_hdr + 1) + end - sizeof(struct sctp_fwdtsn_skip);\
      pos++)
 
 /* External references. */

net/sctp/stream_interleave.c

@@ -1153,8 +1153,8 @@ static void sctp_generate_iftsn(struct sctp_outq *q, __u32 ctsn)
 }
 
 #define _sctp_walk_ifwdtsn(pos, chunk, end) \
-	for (pos = chunk->subh.ifwdtsn_hdr->skip; \
-	     (void *)pos <= (void *)chunk->subh.ifwdtsn_hdr->skip + (end) - \
+	for (pos = (void *)(chunk->subh.ifwdtsn_hdr + 1); \
+	     (void *)pos <= (void *)(chunk->subh.ifwdtsn_hdr + 1) + (end) - \
 			    sizeof(struct sctp_ifwdtsn_skip); pos++)
 
 #define sctp_walk_ifwdtsn(pos, ch) \