mISDN: fix OOM condition for sending queued I-Frames

The old code did not check the return value of skb_clone().
The extra skb_clone() is not needed at all, if using skb_realloc_headroom()
instead, which gives us a private copy with enough headroom as well.
We need to requeue the original skb if the call failed, because we cannot
inform upper layers about the data loss. Restructure the code to minimise
rollback effort if it happens.
This fix kernel bug #86091

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>