net: rmnet_usb: Duplicate skb when there is no tailroom

If the skb to be transmitted has no trailroom for mux padding,
then copy the skb into a new expanded skb with tailroom.

CRs-Fixed: 545483
Change-Id: I6d41c7b80246aac928e8466091cc3e925ecf550c
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>