net: Fix OOPS in skb_seq_read().

It oopsd for me in skb_seq_read. addr2line said it was
linux-2.6/net/core/skbuff.c:2228, which is this line:


	while (st->frag_idx < skb_shinfo(st->cur_skb)->nr_frags) {


I added some printks in there and it looks like we hit this:

        } 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;
        }



Actually I did some testing and added a few printks and found that the
st->cur_skb->data was 0 and hence the ptr used by iscsi_tcp was null.
This caused the kernel panic.

 	if (abs_offset < block_limit) {
-		*data = st->cur_skb->data + abs_offset;
+		*data = st->cur_skb->data + (abs_offset - st->stepped_offset);

I enabled the debug_tcp and with a few printks found that the code did
not go to the next_skb label and could find that the sequence being
followed was this -

It hit this if condi...