net: fix skb_defer_rx_timestamp()

After commit c1f19b5 (net: support time stamping in phy devices.),
kernel might crash if CONFIG_NETWORK_PHY_TIMESTAMPING=y and
skb_defer_rx_timestamp() handles a packet without an ethernet header.

Fixes kernel bugzilla #24102

Reference: https://bugzilla.kernel.org/show_bug.cgi?id=24102
Reported-and-tested-by: Andrew Watts <akwatts@ymail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/timestamping.c

@@ -96,11 +96,13 @@ bool skb_defer_rx_timestamp(struct sk_buff *skb)
 	struct phy_device *phydev;
 	unsigned int type;
 
-	skb_push(skb, ETH_HLEN);
+	if (skb_headroom(skb) < ETH_HLEN)
+		return false;
+	__skb_push(skb, ETH_HLEN);
 
 	type = classify(skb);
 
-	skb_pull(skb, ETH_HLEN);
+	__skb_pull(skb, ETH_HLEN);
 
 	switch (type) {
 	case PTP_CLASS_V1_IPV4: