Add ib_uverbs_flow_spec_mpls to define a rule to match the MPLS
protocol.

The spec includes the generic specs header, type, size and reserved
fields while the filter itself is defined as ib_uverbs_flow_mpls_filter
and includes a single 32bit field named 'label' which consists of:
Bits 0:19  - The MPLS label.
Bits 20:22 - Traffic class field.
Bit  23    - Bottom of stack bit.
Bits 24:31 - Time to live (TTL) field.
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>