ipv6: lockless IPV6_UNICAST_HOPS implementation

Some np->hop_limit accesses are racy, when socket lock is not held.

Add missing annotations and switch to full lockless implementation.

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