net/ipv6: propagate net.ipv6.conf.all.addr_gen_mode to devices

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: David Ahern <dsahern@gmail.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>