[PPPOE]: Missing result check in __pppoe_xmit().

skb_clone() may fail, we should check the result.

Coverity CID: 1215.

Signed-off-by: Florin Malita <fmalita@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>