vlan: should take into account needed_headroom

Commit c95b819ad7 (gre: Use needed_headroom)
made gre use needed_headroom instead of hard_header_len

This uncover a bug in vlan code.

We should make sure vlan devices take into account their
real_dev->needed_headroom or we risk a crash in ipgre_header(), because
we dont have enough room to push IP header in skb.

Reported-by: Diddi Oscarsson <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Patrick McHardy <[e-mail]>
Cc: Herbert Xu <[e-mail]>
Acked-by: Herbert Xu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/8021q/vlan_dev.c

···
 	dev->fcoe_ddp_xid = real_dev->fcoe_ddp_xid;
 #endif
 
+	dev->needed_headroom = real_dev->needed_headroom;
 	if (real_dev->features & NETIF_F_HW_VLAN_TX) {
 		dev->header_ops      = real_dev->header_ops;
 		dev->hard_header_len = real_dev->hard_header_len;