vxlan: vxlan dev should inherit lowerdev's gso_max_size

vxlan dev currently ignores lowerdev's gso_max_size, which adversely
affects TSO performance of liquidio if it's the lowerdev.  Egress TCP
packets' skb->len often exceed liquidio's advertised gso_max_size.  This
may happen on other NIC drivers.

Fix it by assigning lowerdev's gso_max_size to that of vxlan dev.  Might as
well do likewise for gso_max_segs.

Single flow TSO throughput of liquidio as lowerdev (using iperf3):

    Before the patch:    139 Mbps
    After the patch :   8.68 Gbps
    Percent increase:  6,144 %
Signed-off-by: NFelix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: NSatanand Burla <satananda.burla@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -2926,6 +2926,11 @@ static int vxlan_dev_configure(struct net *src_net, struct net_device *dev,
 		return -EINVAL;
 	}
 
+	if (lowerdev) {
+		dev->gso_max_size = lowerdev->gso_max_size;
+		dev->gso_max_segs = lowerdev->gso_max_segs;
+	}
+
 	if (conf->mtu) {
 		int max_mtu = ETH_MAX_MTU;