ipv4: Fix incorrect table ID in IOCTL path

Commit f96a3d7455 ("ipv4: Fix incorrect route flushing when source
address is deleted") started to take the table ID field in the FIB info
structure into account when determining if two structures are identical
or not. This field is initialized using the 'fc_table' field in the
route configuration structure, which is not set when adding a route via
IOCTL.

The above can result in user space being able to install two identical
routes that only differ in the table ID field of their associated FIB
info.

Fix by initializing the table ID field in the route configuration
structure in the IOCTL path.

Before the fix:

 # ip route add default via 192.0.2.2
 # route add default gw 192.0.2.2
 # ip -4 r show default
 # default via 192.0.2.2 dev dummy10
 # default via 192.0.2.2 dev dummy10

After the fix:

 # ip route add default via 192.0.2.2
 # route add default gw 192.0.2.2
 SIOCADDRT: File exists
 # ip -4 r show default
 default via 192.0.2.2 dev dummy10

Audited the code paths to ensure there are no other paths that do not
properly initialize the route configuration structure when installing a
route.

Fixes: 5a56a0b3a4 ("net: Don't delete routes in different VRFs")
Fixes: f96a3d7455 ("ipv4: Fix incorrect route flushing when source address is deleted")
Reported-by: gaoxingwang <gaoxingwang1@huawei.com>
Link: https://lore.kernel.org/netdev/20230314144159.2354729-1-gaoxingwang1@huawei.com/
Tested-by: gaoxingwang <gaoxingwang1@huawei.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230315124009.4015212-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/fib_frontend.c

@@ -576,6 +576,9 @@ static int rtentry_to_fib_config(struct net *net, int cmd, struct rtentry *rt,
 			cfg->fc_scope = RT_SCOPE_UNIVERSE;
 	}
 
+	if (!cfg->fc_table)
+		cfg->fc_table = RT_TABLE_MAIN;
+
 	if (cmd == SIOCDELRT)
 		return 0;