[NET]: Avoid unnecessary cloning for ingress filtering

As it is we always invoke pt_prev before ing_filter, even if there are no
ingress filters attached.  This can cause unnecessary cloning in pt_prev.

This patch changes it so that we only invoke pt_prev if there are ingress
filters attached.

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