net/ipv6: allow sysctl to change link-local address generation mode

The address generation mode for IPv6 link-local can only be configured
by netlink messages. This patch adds the ability to change the address
generation mode via sysctl.

v1 -> v2
Removed the rtnl lock and switch to use RCU lock to iterate through
the netdev list.

v2 -> v3
Removed the addrgenmode variable from the idev structure and use the
systcl storage for the flag.

Simplifed the logic for sysctl handling by removing the supported
for all operation.

Added support for more types of tunnel interfaces for link-local
address generation.

Based the patches from net-next.

v3 -> v4
Removed unnecessary whitespace changes.

Signed-off-by: Felix Jia <felix.jia@alliedtelesis.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 671d014..71be5b3 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -69,6 +69,7 @@ struct ipv6_devconf {
 	__s32		seg6_require_hmac;
 #endif
 	__u32		enhanced_dad;
+	__u32		addr_gen_mode;
 
 	struct ctl_table_header *sysctl_header;
 };