af_iucv: use sk functions to modify sk->sk_ack_backlog

Instead of modifying sk->sk_ack_backlog directly, use respective
socket functions.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>