ofproto-dpif: MPLS recirculation

In some cases an pop MPLS action changes a packet to be a non-mpls packet.
In this case subsequent any L3+ actions require access to portions
of the packet which were not decoded as they were opaque when the
packet was MPLS. Allow such actions to be translated by
first recirculating the packet.

Co-authored-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>
ofp-actions: Allow pop_mpls on MPLS packets

With recirculation in place this should be safe.

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

Co-authored-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>