net: Use passed in table for nexthop lookups

If a user passes in a table for new routes use that table for nexthop
lookups. Specifically, this solves the case where a connected route does
not exist in the main table, but only another table and then a subsequent
route is added with a next hop using the connected route. ie.,

$ ip route ls
default via 10.0.2.2 dev eth0
10.0.2.0/24 dev eth0  proto kernel  scope link  src 10.0.2.15
169.254.0.0/16 dev eth0  scope link  metric 1003
192.168.56.0/24 dev eth1  proto kernel  scope link  src 192.168.56.51

$ ip route ls table 10
1.1.1.0/24 dev eth2  scope link

Without this patch adding a nexthop route fails:

$ ip route add table 10 2.2.2.0/24 via 1.1.1.10
RTNETLINK answers: Network is unreachable

With this patch the route is added successfully.
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_semantics.c

@@ -691,6 +691,7 @@ static int fib_check_nh(struct fib_config *cfg, struct fib_info *fi,
 		}
 		rcu_read_lock();
 		{
+			struct fib_table *tbl = NULL;
 			struct flowi4 fl4 = {
 				.daddr = nh->nh_gw,
 				.flowi4_scope = cfg->fc_scope + 1,
@@ -701,6 +702,14 @@ static int fib_check_nh(struct fib_config *cfg, struct fib_info *fi,
 			/* It is not necessary, but requires a bit of thinking */
 			if (fl4.flowi4_scope < RT_SCOPE_LINK)
 				fl4.flowi4_scope = RT_SCOPE_LINK;
+
+			if (cfg->fc_table)
+				tbl = fib_get_table(net, cfg->fc_table);
+
+			if (tbl)
+				err = fib_table_lookup(tbl, &fl4, &res,
+						   FIB_LOOKUP_IGNORE_LINKSTATE);
+			else
 				err = fib_lookup(net, &fl4, &res,
 						 FIB_LOOKUP_IGNORE_LINKSTATE);
 			if (err) {