rxrpc: Fix incorrect conditional on IPV6

The udpv6_encap_enable() function is part of the ipv6 code, and if that is
configured as a loadable module and rxrpc is built in then a build failure
will occur because the conditional check is wrong:

  net/rxrpc/local_object.o: In function `rxrpc_lookup_local':
  local_object.c:(.text+0x2688): undefined reference to `udpv6_encap_enable'

Use the correct config symbol (CONFIG_AF_RXRPC_IPV6) in the conditional
check rather than CONFIG_IPV6 as that will do the right thing.

Fixes: 5271953c ("rxrpc: Use the UDP encap_rcv hook")
Reported-by: kbuild-all@01.org
Reported-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/rxrpc/local_object.c

@@ -139,7 +139,7 @@ static int rxrpc_open_socket(struct rxrpc_local *local, struct net *net)
 	udp_sk(usk)->gro_complete = NULL;
 
 	udp_encap_enable();
-#if IS_ENABLED(CONFIG_IPV6)
+#if IS_ENABLED(CONFIG_AF_RXRPC_IPV6)
 	if (local->srx.transport.family == AF_INET6)
 		udpv6_encap_enable();
 #endif