ipv6: lockless IPV6_RECVERR_RFC4884 implementation

Move np->recverr_rfc4884 to an atomic flag to fix data-races.

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