It is possible for the headroom to be smaller then the
hard_header_len for a short period of time after toggling
the vlan offload setting.

This is not a hard error and skb_cow_head is called in
__vlan_put_tag() to resolve this.
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>