[NET]: Make skb_seq_read unmap the last fragment

Having walked through the entire skbuff, skb_seq_read would leave the
last fragment mapped.  As a consequence, the unwary caller would leak
kmaps, and proceed with preempt_count off by one. The only (kind of
non-intuitive) workaround is to use skb_seq_read_abort.

This patch makes sure skb_seq_read always unmaps frag_data after
having cycled through the skb's paged part.
Signed-off-by: Olaf Kirch <olaf.kirch@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -1706,6 +1706,11 @@ next_skb:
 		st->stepped_offset += frag->size;
 	}
 
+	if (st->frag_data) {
+		kunmap_skb_frag(st->frag_data);
+		st->frag_data = NULL;
+	}
+
 	if (st->cur_skb->next) {
 		st->cur_skb = st->cur_skb->next;
 		st->frag_idx = 0;