inet: Add getsockopt support for IP_ROUTER_ALERT and IPV6_ROUTER_ALERT

Currently getsockopt does not support IP_ROUTER_ALERT and
IPV6_ROUTER_ALERT, and we are unable to get the values of these two
socket options through getsockopt.

This patch adds getsockopt support for IP_ROUTER_ALERT and
IPV6_ROUTER_ALERT.

Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>