mpls: Save the L3 header offset in the meta-data before label imposition
mpls: Save the L3 header offset in the meta-data before label imposition

Type: improvement

Subsequent features in the data-path can thus easily find the l3 header
without parsing the label stack.

Signed-off-by: Neale Ranns <c8ca090d26e35fc94cb9c8fe2fb2efcfa843d561@graphiant.com>
Change-Id: I26f7d4bbe9186aeb8654706579c72424e8ecca2c

FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp