ISDN: fix OOM condition for sending queued I-Frames

The skb_clone() return value was not checked and the skb_realloc_headroom()
usage was wrong, the old skb was not freed. It turned out, that the
skb_clone is not needed at all, the skb_realloc_headroom() will create a
private copy with enough headroom and the original SKB can be used for the
ACK queue.
We need to requeue the original skb if the call failed, since the upper
layer cannot be informed about memory shortage.

Thanks to Insu Yun <wuninsu@gmail.com> to remind me on this issue.

Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>