ipv6: lockless IPV6_MULTICAST_HOPS implementation

This fixes data-races around np->mcast_hops,
and make IPV6_MULTICAST_HOPS lockless.

Note that np->mcast_hops is never negative,
thus can fit an u8 field instead of s16.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>