flow_dissector: Add flag to stop parsing at L3

Add an input flag to flow dissector on rather dissection should be
stopped when an L3 packet is encountered. This would be useful if a
caller just wanted to get IP addresses of the outermost header (e.g.
to do an L3 hash).
Signed-off-by: NTom Herbert <tom@herbertland.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/flow_dissector.h

@@ -125,6 +125,7 @@ enum flow_dissector_key_id {
 };
 
 #define FLOW_DISSECTOR_F_PARSE_1ST_FRAG		BIT(0)
+#define FLOW_DISSECTOR_F_STOP_AT_L3		BIT(1)
 
 struct flow_dissector_key {
 	enum flow_dissector_key_id key_id;

net/core/flow_dissector.c

@@ -200,6 +200,9 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 			}
 		}
 
+		if (flags & FLOW_DISSECTOR_F_STOP_AT_L3)
+			goto out_good;
+
 		break;
 	}
 	case htons(ETH_P_IPV6): {
@@ -238,6 +241,9 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 			}
 		}
 
+		if (flags & FLOW_DISSECTOR_F_STOP_AT_L3)
+			goto out_good;
+
 		break;
 	}
 	case htons(ETH_P_8021AD):