net/af_iucv: build proper skbs for HiperTransport

The HiperSockets-based transport path in af_iucv is still too closely
entangled with qeth.
With commit a647a025 ("s390/qeth: speed-up L3 IQD xmit"), the
relevant xmit code in qeth has begun to use skb_cow_head(). So to avoid
unnecessary skb head expansions, af_iucv must learn to
1) respect dev->needed_headroom when allocating skbs, and
2) drop the header reference before cloning the skb.

While at it, also stop hard-coding the LL-header creation stage and just
use the appropriate helper.

Fixes: a647a025

 ("s390/qeth: speed-up L3 IQD xmit")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>