ipv6: Move ipv6 stubs to a separate header file

The number of stubs is growing and has nothing to do with addrconf.
Move the definition of the stubs to a separate header file and update
users. In the move, drop the vxlan specific comment before ipv6_stub.

Code move only; no functional change intended.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
index d76dfed..5994d54 100644
--- a/drivers/net/vxlan.c
+++ b/drivers/net/vxlan.c
@@ -20,6 +20,7 @@
 #include <linux/ethtool.h>
 #include <net/arp.h>
 #include <net/ndisc.h>
+#include <net/ipv6_stubs.h>
 #include <net/ip.h>
 #include <net/icmp.h>
 #include <net/rtnetlink.h>