veth: Ensure eth header is in skb's linear part

In the Linux kernel, the following vulnerability has been resolved:

veth: Ensure eth header is in skb's linear part

After feeding a decapsulated packet to a veth device with act_mirred,
skb_headlen() may be 0. But veth_xmit() calls __dev_forward_skb(),
which expects at least ETH_HLEN byte of linear data (as
__dev_forward_skb2() calls eth_type_trans(), which pulls ETH_HLEN bytes
unconditionally).

Use pskb_may_pull() to ensure veth_xmit() respects this constraint.

kernel BUG at include/linux/skbuff.h:2328!
RIP: 0010:eth_type_trans+0xcf/0x140
Call Trace:
<IRQ>
__dev_forward_skb2+0xe3/0x160
veth_xmit+0x6e/0x250 [veth]
dev_hard_start_xmit+0xc7/0x200
__dev_queue_xmit+0x47f/0x520
? skb_ensure_writable+0x85/0xa0
? skb_mpls_pop+0x98/0x1c0
tcf_mirred_act+0x442/0x47e [act_mirred]
tcf_action_exec+0x86/0x140
fl_classify+0x1d8/0x1e0 [cls_flower]
? dma_pte_clear_level+0x129/0x1a0
? dma_pte_clear_level+0x129/0x1a0
? prb_fill_curr_block+0x2f/0xc0
? skb_copy_bits+0x11a/0x220
__tcf_classify+0x58/0x110
tcf_classify_ingress+0x6b/0x140
__netif_receive_skb_core.constprop.0+0x47d/0xfd0
? __iommu_dma_unmap_swiotlb+0x44/0x90
__netif_receive_skb_one_core+0x3d/0xa0
netif_receive_skb+0x116/0x170
be_process_rx+0x22f/0x330 [be2net]
be_poll+0x13c/0x370 [be2net]
__napi_poll+0x2a/0x170
net_rx_action+0x22f/0x2f0
__do_softirq+0xca/0x2a8
__irq_exit_rcu+0xc1/0xe0
common_interrupt+0x83/0xa0