rxrpc: Make IPv6 support conditional on CONFIG_IPV6

Add CONFIG_AF_RXRPC_IPV6 and make the IPv6 support code conditional on it.
This is then made conditional on CONFIG_IPV6.

Without this, the following can be seen:

   net/built-in.o: In function `rxrpc_init_peer':
>> peer_object.c:(.text+0x18c3c8): undefined reference to `ip6_route_output_flags'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/rxrpc/Kconfig b/net/rxrpc/Kconfig
index 784c531..13396c7 100644
--- a/net/rxrpc/Kconfig
+++ b/net/rxrpc/Kconfig
@@ -19,6 +19,13 @@
 
 	  See Documentation/networking/rxrpc.txt.
 
+config AF_RXRPC_IPV6
+	bool "IPv6 support for RxRPC"
+	depends on (IPV6 = m && AF_RXRPC = m) || (IPV6 = y && AF_RXRPC)
+	help
+	  Say Y here to allow AF_RXRPC to use IPV6 UDP as well as IPV4 UDP as
+	  its network transport.
+
 
 config AF_RXRPC_DEBUG
 	bool "RxRPC dynamic debugging"