net/ipv6: don't reinitialize ndev->cnf.addr_gen_mode on new inet6_dev

The value has already been copied from this netns's devconf_dflt, it
shouldn't be reset to the global kernel default.

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