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

Fixes: e314dbdc ("[NET]: Virtual ethernet device driver.")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>