ppp_generic: pull 2 bytes so that PPP_PROTO(skb) is valid

In ppp_input(), PPP_PROTO(skb) may refer to invalid data in the skb.

If this happens and (proto >= 0xc000 || proto == PPP_CCPFRAG) then
the packet is passed directly to pppd.

This occurs frequently when using PPPoE with an interface MTU
greater than 1500 because the skb is more likely to be non-linear.

The next 2 bytes need to be pulled in ppp_input(). The pull of 2
bytes in ppp_receive_frame() has been removed as it is no longer
required.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>