inet: implement lockless IP_MULTICAST_TTL

inet->mc_ttl can be read locklessly.

Implement proper lockless reads and writes to inet->mc_ttl

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