[IPV4] ip_gre: set mac_header correctly in receive path

mac_header update in ipgre_recv() was incorrectly changed to
skb_reset_mac_header() when it was introduced.

Signed-off-by: Timo Teras <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>