vxlan: Update hard_header_len based on lowerdev when instantiating VXLAN

In the event of a VXLAN device being linked to a device that has a
hard_header_len greater than that of standard ethernet we could end up with
the hard_header_len not being large enough for outgoing frames.  In order to
prevent this we should update the length when a lowerdev is provided.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>