mpls: Fix allowed protocols for mpls gso

MPLS and Tunnel GSO does not work together.  Reject packet which
request such GSO.

Fixes: 0d89d203 ("MPLS: Add limited GSO support").
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>