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: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>