tcp_metrics: validate source addr length

I don't see anything checking that TCP_METRICS_ATTR_SADDR_IPV4
is at least 4 bytes long, and the policy doesn't have an entry
for this attribute at all (neither does it for IPv6 but v6 is
manually validated).
Reviewed-by: Eric Dumazet <edumazet@google.com>
Fixes: 3e7013dd ("tcp: metrics: Allow selective get/del of tcp-metrics based on src IP")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_metrics.c

@@ -619,6 +619,7 @@ static const struct nla_policy tcp_metrics_nl_policy[TCP_METRICS_ATTR_MAX + 1] =
 	[TCP_METRICS_ATTR_ADDR_IPV4]	= { .type = NLA_U32, },
 	[TCP_METRICS_ATTR_ADDR_IPV6]	= { .type = NLA_BINARY,
 					    .len = sizeof(struct in6_addr), },
+	[TCP_METRICS_ATTR_SADDR_IPV4]	= { .type = NLA_U32, },
 	/* Following attributes are not received for GET/DEL,
 	 * we keep them for reference
 	 */