[NET] ppp: Remove unnecessary pskb_may_pull

In ppp_receive_nonmp_frame, we call pskb_may_pull(skb, skb->len) if the
tailroom is >= 124.  This is pointless because this pskb_may_pull is only
needed if the skb is non-linear.  However, if it is non-linear then the
tailroom would be zero.

So it can be safely removed.

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