net: convert __IPTUNNEL_XMIT() to an inline function

__IPTUNNEL_XMIT() is an ugly macro, convert it to a static
inline function, so make it more readable.

IPTUNNEL_XMIT() is unused, just remove it.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
index 8aca888..9814d67 100644
--- a/drivers/net/vxlan.c
+++ b/drivers/net/vxlan.c
@@ -769,7 +769,7 @@
 
 	vxlan_set_owner(dev, skb);
 
-	/* See __IPTUNNEL_XMIT */
+	/* See iptunnel_xmit() */
 	skb->ip_summed = CHECKSUM_NONE;
 	ip_select_ident(iph, &rt->dst, NULL);