net: ipv6: Fix processing of RAs in presence of VRF

rt6_add_route_info and rt6_add_dflt_router were updated to pull the FIB
table from the device index, but the corresponding rt6_get_route_info
and rt6_get_dflt_router functions were not leading to the failure to
process RA's:

    ICMPv6: RA: ndisc_router_discovery failed to add default route

Fix the 'get' functions by using the table id associated with the
device when applicable.

Also, now that default routes can be added to tables other than the
default table, rt6_purge_dflt_routers needs to be updated as well to
look at all tables. To handle that efficiently, add a flag to the table
denoting if it is has a default route via RA.

Fixes: ca254490c8 ("net: Add VRF support to IPv6 stack")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip6_fib.h

@@ -230,6 +230,8 @@ struct fib6_table {
 	rwlock_t		tb6_lock;
 	struct fib6_node	tb6_root;
 	struct inet_peer_base	tb6_peers;
+	unsigned int		flags;
+#define RT6_TABLE_HAS_DFLT_ROUTER	BIT(0)
 };
 
 #define RT6_TABLE_UNSPEC	RT_TABLE_UNSPEC