net/ipv6: reserve room for IFLA_INET6_ADDR_GEN_MODE

inet6_ifla6_size() is called to check how much space is needed by
inet6_fill_link_af() and inet6_fill_ifinfo(), both of which include
the IFLA_INET6_ADDR_GEN_MODE attribute. Reserve some room for it.

Fixes: bc91b0f0 ("ipv6: addrconf: implement address generation modes")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>