tunnels: support MPLS over IPv4 tunnels

Extend tunnel support to MPLS over IPv4.  The implementation extends the
existing differentiation between IPIP and IPv6 over IPv4 to also cover MPLS
over IPv4.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Dinan Gunawardena <dinan.gunawardena@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>