net: Add docbook description for 'mtu' arg to skb_gso_validate_mtu()

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index b6e0f95..e7ec6d3 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4399,6 +4399,7 @@
  * skb_gso_validate_mtu - Return in case such skb fits a given MTU
  *
  * @skb: GSO skb
+ * @mtu: MTU to validate against
  *
  * skb_gso_validate_mtu validates if a given skb will fit a wanted MTU
  * once split.