octeontx2-pf: Add error handling to VLAN unoffload handling

otx2_sq_append_skb makes used of __vlan_hwaccel_push_inside()
to unoffload VLANs - push them from skb meta data into skb data.
However, it omitts a check for __vlan_hwaccel_push_inside()
returning NULL.

Found by inspection based on [1] and [2].
Compile tested only.

[1] Re: [PATCH net-next v1] net: stmmac: Enable TSO on VLANs
    https://lore.kernel.org/all/ZmrN2W8Fye450TKs@shell.armlinux.org.uk/
[2] Re: [PATCH net-next v2] net: stmmac: Enable TSO on VLANs
    https://lore.kernel.org/all/CANn89i+11L5=tKsa7V7Aeyxaj6nYGRwy35PAbCRYJ73G+b25sg@mail.gmail.com/

Fixes: fd9d7859 ("octeontx2-pf: Implement ingress/egress VLAN offload")
Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/octeontx2/nic/otx2_txrx.c

@@ -1174,8 +1174,11 @@ bool otx2_sq_append_skb(struct net_device *netdev, struct otx2_snd_queue *sq,
 
 	if (skb_shinfo(skb)->gso_size && !is_hw_tso_supported(pfvf, skb)) {
 		/* Insert vlan tag before giving pkt to tso */
-		if (skb_vlan_tag_present(skb))
+		if (skb_vlan_tag_present(skb)) {
 			skb = __vlan_hwaccel_push_inside(skb);
+			if (!skb)
+				return true;
+		}
 		otx2_sq_append_tso(pfvf, sq, skb, qidx);
 		return true;
 	}