net/ipv6: convert skip_notify_on_dev_down sysctl to u8

Save a bit a space, and could help future sysctls to
use the same pattern.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>