net/mlx5: add option to configure FCS or decapsulation

net/mlx5: add option to configure FCS or decapsulation

There are some limitations on some NICs (at least on ConnectX-6 Dx
and BlueField 2) with supporting FCS (frame checksum) scattering for
the tunnel decapsulated packets.

For the case only one of the features can be supported in the same time,
and the new devarg "decap_en" is introduced to provide the choice to the
users.

If FCS scattering feature is not supposed to be engaged by application,
this new devarg should be specified as "decap_en=0", forcing the FCS
feature enable and rejecting tunnel decap actions in the rte_flow engine.
If FCS scatter is not needed and application supposes to use tunnel
decapsulation in rte_flow, the devarg can be omitted or set to non-zero
value (this is default settings).

Signed-off-by: Suanming Mou <suanmingm@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>