af_iucv: fix recvmsg by replacing skb_pull() function

When receiving data messages, the "BUG_ON(skb->len < skb->data_len)" in
the skb_pull() function triggers a kernel panic.

Replace the skb_pull logic by a per skb offset as advised by
Eric Dumazet.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>