ipv4: add description about martian source

When multiple containers are running in the environment and multiple
macvlan network port are configured in each container, a lot of martian
source prints will appear after martian_log is enabled. they are almost
the same, and printed by net_warn_ratelimited. Each arp message will
trigger this print on each network port.

Such as:
IPv4: martian source 173.254.95.16 from 173.254.100.109,
on dev eth0
ll header: 00000000: ff ff ff ff ff ff 40 00 ad fe 64 6d
08 06        ......@...dm..
IPv4: martian source 173.254.95.16 from 173.254.100.109,
on dev eth1
ll header: 00000000: ff ff ff ff ff ff 40 00 ad fe 64 6d
08 06        ......@...dm..

There is no description of this kind of source in the RFC1812.
Signed-off-by: Zhang Yunkai <zhang.yunkai@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -436,6 +436,9 @@ int fib_validate_source(struct sk_buff *skb, __be32 src, __be32 dst,
 		if (net->ipv4.fib_has_custom_local_routes ||
 		    fib4_has_custom_rules(net))
 			goto full_check;
+		/* Within the same container, it is regarded as a martian source,
+		 * and the same host but different containers are not.
+		 */
 		if (inet_lookup_ifaddr_rcu(net, src))
 			return -EINVAL;