openvswitch: use mpls_hdr

skb_mpls_header is equivalent to mpls_hdr now. Use the existing helper
instead.

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>