inet: implement lockless IP_MINTTL

inet->min_ttl is already read with READ_ONCE().

Implementing IP_MINTTL socket option set/read
without holding the socket lock is easy.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>