openvswitch: mpls: set network header correctly on key extract

After the 48d2ab60 ("net: mpls: Fixups for GSO"), MPLS handling in
openvswitch was changed to have network header pointing to the start of the
MPLS headers and inner_network_header pointing after the MPLS headers.

However, key_extract was missed by the mentioned commit, causing incorrect
headers to be set when a MPLS packet just enters the bridge or after it is
recirculated.

Fixes: 48d2ab60

 ("net: mpls: Fixups for GSO")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>