net: Unmap fragment page once iterator is done

Callers of skb_seq_read() are currently forced to call skb_abort_seq_read()
even when consuming all the data because the last call to skb_seq_read (the
one that returns 0 to indicate the end) fails to unmap the last fragment page.

With this patch callers will be allowed to traverse the SKB data by calling
skb_prepare_seq_read() once and repeatedly calling skb_seq_read() as originally
intended (and documented in the original commit 677e90ed), that is, only call
skb_abort_seq_read() if the sequential read is actually aborted.
Signed-off-by: NWedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/scsi/libiscsi_tcp.c

@@ -906,7 +906,6 @@ int iscsi_tcp_recv_skb(struct iscsi_conn *conn, struct sk_buff *skb,
 			ISCSI_DBG_TCP(conn, "no more data avail. Consumed %d\n",
 				      consumed);
 			*status = ISCSI_TCP_SKB_DONE;
-			skb_abort_seq_read(&seq);
 			goto skb_done;
 		}
 		BUG_ON(segment->copied >= segment->size);

net/batman-adv/main.c

@@ -473,7 +473,6 @@ __be32 batadv_skb_crc32(struct sk_buff *skb, u8 *payload_ptr)
 		crc = crc32c(crc, data, len);
 		consumed += len;
 	}
-	skb_abort_seq_read(&st);
 
 	return htonl(crc);
 }

net/core/skbuff.c

@@ -2541,8 +2541,13 @@ unsigned int skb_seq_read(unsigned int consumed, const u8 **data,
 	unsigned int block_limit, abs_offset = consumed + st->lower_offset;
 	skb_frag_t *frag;
 
-	if (unlikely(abs_offset >= st->upper_offset))
+	if (unlikely(abs_offset >= st->upper_offset)) {
+		if (st->frag_data) {
+			kunmap_atomic(st->frag_data);
+			st->frag_data = NULL;
+		}
 		return 0;
+	}
 
 next_skb:
 	block_limit = skb_headlen(st->cur_skb) + st->stepped_offset;