[PPP] pppoe: Fix double-free on skb after transmit failure

When I got rid of the second packet in __pppoe_xmit I created
a double-free on the skb because of the goto abort on failure.
This patch removes that.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>