[PPP] pppoe: Fill in header directly in __pppoe_xmit

This patch removes the hdr variable (which is copied into the skb)
and instead sets the header directly in the skb.

It also uses __skb_push instead of skb_push since we've just checked
using skb_cow for enough head room.

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