[IPV6] NDISC: Don't rely on node-type hint from L2 unless required.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/Kconfig

@@ -168,6 +168,7 @@ config IPV6_SIT
 	tristate "IPv6: IPv6-in-IPv4 tunnel (SIT driver)"
 	depends on IPV6
 	select INET_TUNNEL
+	select IPV6_NDISC_NODETYPE
 	default y
 	---help---
 	  Tunneling means encapsulating data of one protocol type within
@@ -178,6 +179,9 @@ config IPV6_SIT
 
 	  Saying M here will produce a module called sit.ko. If unsure, say Y.
 
+config IPV6_NDISC_NODETYPE
+	bool
+
 config IPV6_TUNNEL
 	tristate "IPv6: IP-in-IPv6 tunnel (RFC2473)"
 	select INET6_TUNNEL