vxlan: set mac_header correctly in GPE mode

For VXLAN-GPE, the interface is ARPHRD_NONE, thus we need to reset
mac_header after pulling the outer header.

v2: Put the code to the existing conditional block as suggested by
    Shmulik Ladkani.

Fixes: e1e5314d

 ("vxlan: implement GPE")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>