[BUG] ethernet:enic: A use after free bug in enic_hard_start_xmit

[lyl2019]

Hi, maintainers.
    Our code analyzer reported a uaf bug, but it is a little
difficult for me to fix it directly.

File: drivers/net/ethernet/cisco/enic/enic_main.c
In enic_hard_start_xmit, it calls enic_queue_wq_skb(). Inside
enic_queue_wq_skb, if some error happens, the skb will be freed
by dev_kfree_skb(skb). But the freed skb is still used in 
skb_tx_timestamp(skb).

```
	enic_queue_wq_skb(enic, wq, skb);// skb could freed here

	if (vnic_wq_desc_avail(wq) < MAX_SKB_FRAGS + ENIC_DESC_MAX_SPLITS)
		netif_tx_stop_queue(txq);
	skb_tx_timestamp(skb); // freed skb is used here.
```
Bug introduced by fb7516d42478e ("enic: add sw timestamp support").

Thanks for your time, looking forwarding to your reply.
Lv Yunlong <lyl2019@xxxxxxxxxxxxxxxx>