ofproto-dpif-xlate: Do not push LSE to flow if its label stack is full

If a pop is composed on a flow whose label stack is already full
it should not call flow_push_mpls() as that function can't do
anything useful (it aborts) given that the label stack is full.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>
ofproto-dpif: Test for A MPLS pop on a packet whose label stack going to userspace

Update the test for an MPLS pop on on a packet whose label stack going to
userspace to test that it goes to userspace, which is currently the case,
rather than being dropped, which was the case earlier.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>