net: Fix frag_list handling in skb_seq_read

The frag_list handling was broken in skb_seq_read:

1) We didn't add the stepped offset when looking at the head
are of fragments other than the first.

2) We didn't take the stepped offset away when setting the data
pointer in the head area.

3) The frag index wasn't reset.

This patch fixes both issues.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -2212,10 +2212,10 @@ unsigned int skb_seq_read(unsigned int consumed, const u8 **data,
 		return 0;
 
 next_skb:
-	block_limit = skb_headlen(st->cur_skb);
+	block_limit = skb_headlen(st->cur_skb) + st->stepped_offset;
 
 	if (abs_offset < block_limit) {
-		*data = st->cur_skb->data + abs_offset;
+		*data = st->cur_skb->data + (abs_offset - st->stepped_offset);
 		return block_limit - abs_offset;
 	}
 
@@ -2257,6 +2257,7 @@ unsigned int skb_seq_read(unsigned int consumed, const u8 **data,
 	} else if (st->root_skb == st->cur_skb &&
 		   skb_shinfo(st->root_skb)->frag_list) {
 		st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
+		st->frag_idx = 0;
 		goto next_skb;
 	}