net/ipv4: Simplify fib_multipath_hash with optional flow keys

As of commit e37b1e97 ("ipv6: route: dissect flow in input path if
fib rules need it") fib_multipath_hash takes an optional flow keys. If
non-NULL it means the skb has already been dissected. If not set, then
fib_multipath_hash needs to call skb_flow_dissect_flow_keys.

Simplify the logic by setting flkeys to the local stack variable keys.
Simplifies fib_multipath_hash by only have 1 set of instructions
setting hash_keys.
Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1810,24 +1810,20 @@ int fib_multipath_hash(const struct net *net, const struct flowi4 *fl4,
 			/* short-circuit if we already have L4 hash present */
 			if (skb->l4_hash)
 				return skb_get_hash_raw(skb) >> 1;
+
 			memset(&hash_keys, 0, sizeof(hash_keys));
 
-			if (flkeys) {
+			if (!flkeys) {
+				skb_flow_dissect_flow_keys(skb, &keys, flag);
+				flkeys = &keys;
+			}
+
 			hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
 			hash_keys.addrs.v4addrs.src = flkeys->addrs.v4addrs.src;
 			hash_keys.addrs.v4addrs.dst = flkeys->addrs.v4addrs.dst;
 			hash_keys.ports.src = flkeys->ports.src;
 			hash_keys.ports.dst = flkeys->ports.dst;
 			hash_keys.basic.ip_proto = flkeys->basic.ip_proto;
-			} else {
-				skb_flow_dissect_flow_keys(skb, &keys, flag);
-				hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
-				hash_keys.addrs.v4addrs.src = keys.addrs.v4addrs.src;
-				hash_keys.addrs.v4addrs.dst = keys.addrs.v4addrs.dst;
-				hash_keys.ports.src = keys.ports.src;
-				hash_keys.ports.dst = keys.ports.dst;
-				hash_keys.basic.ip_proto = keys.basic.ip_proto;
-			}
 		} else {
 			memset(&hash_keys, 0, sizeof(hash_keys));
 			hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;