IB/mlx5: Add support for MPLS flow specification

This patch introduces support for the MPLS flow spec and
allows the creation of rules that are matching on the
MPLS label.

Applying the rule matching depends on the flow specs order and
the location of the MPLS in the spec list as there are different
configurations to be made in the device in the cases of MPLSoGRE
and MPLSoUDP vs. non-encapsulated MPLS.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>