skbuff: Fix not waking applications when errors are enqueued

When errors are enqueued to the error queue via sock_queue_err_skb()
function, it is possible that the waiting application is not notified.

Calling 'sk->sk_data_ready()' would not notify applications that
selected only POLLERR events in poll() (for example).

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: Randy E. Witt <randy.e.witt@intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>