ipv6: export ip6 fragments sysctl to unprivileged users

IPv4 was changed in commit 52a773d6 ("net: Export ip fragment
sysctl to unprivileged users")

The only sysctl that is not per-netns is not used :
ip6frag_secret_interval

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 18dcbe12fe9fca0ab825f7eff993060525ac2503)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>