This aligns the addr_gen_mode sysctl with the expected behavior of the
"all" variant.

Fixes: d35a00b8 ("net/ipv6: allow sysctl to change link-local address generation mode")
Suggested-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NSabrina Dubroca <sd@queasysnail.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>