filter: constify sk_run_filter()

sk_run_filter() doesnt write on skb, change its prototype to reflect
this.

Fix two af_packet comments.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/timestamping.c b/net/core/timestamping.c
index dac7ed6..b124d28 100644
--- a/net/core/timestamping.c
+++ b/net/core/timestamping.c
@@ -26,7 +26,7 @@
 	PTP_FILTER
 };
 
-static unsigned int classify(struct sk_buff *skb)
+static unsigned int classify(const struct sk_buff *skb)
 {
 	if (likely(skb->dev &&
 		   skb->dev->phydev &&