net: Add helper flowi4_init_output().

On-stack initialization via assignment of flow structures are
expensive because GCC emits a memset() to clear the entire
structure out no matter what.

Add a helper for ipv4 output flow key setup which we can use to avoid
the memset.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow.h

@@ -70,6 +70,27 @@ struct flowi4 {
 #define fl4_gre_key		uli.gre_key
 };
 
+static inline void flowi4_init_output(struct flowi4 *fl4, int oif,
+				      __u32 mark, __u8 tos, __u8 scope,
+				      __u8 proto, __u8 flags,
+				      __be32 daddr, __be32 saddr,
+				      __be16 dport, __be32 sport)
+{
+	fl4->flowi4_oif = oif;
+	fl4->flowi4_iif = 0;
+	fl4->flowi4_mark = mark;
+	fl4->flowi4_tos = tos;
+	fl4->flowi4_scope = scope;
+	fl4->flowi4_proto = proto;
+	fl4->flowi4_flags = flags;
+	fl4->flowi4_secid = 0;
+	fl4->daddr = daddr;
+	fl4->saddr = saddr;
+	fl4->fl4_sport = sport;
+	fl4->fl4_dport = dport;
+}
+				      
+
 struct flowi6 {
 	struct flowi_common	__fl_common;
 #define flowi6_oif		__fl_common.flowic_oif