net: Refactor path selection in __ip_route_output_key_hash

VRF device needs the same path selection following lookup to set source
address. Rather than duplicating code, move existing code into a
function that is exported to modules.

Code move only; no functional change.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 54297d3..54e6f45 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2238,21 +2238,7 @@
 		goto make_route;
 	}
 
-#ifdef CONFIG_IP_ROUTE_MULTIPATH
-	if (res.fi->fib_nhs > 1 && fl4->flowi4_oif == 0) {
-		if (mp_hash < 0)
-			mp_hash = fib_multipath_hash(fl4->saddr, fl4->daddr);
-		fib_select_multipath(&res, mp_hash);
-	}
-	else
-#endif
-	if (!res.prefixlen &&
-	    res.table->tb_num_default > 1 &&
-	    res.type == RTN_UNICAST && !fl4->flowi4_oif)
-		fib_select_default(fl4, &res);
-
-	if (!fl4->saddr)
-		fl4->saddr = FIB_RES_PREFSRC(net, res);
+	fib_select_path(net, &res, fl4, mp_hash);
 
 	dev_out = FIB_RES_DEV(res);
 	fl4->flowi4_oif = dev_out->ifindex;