mlx5: get rid of the WR structure in Rx queue elements

mlx5: get rid of the WR structure in Rx queue elements

Removing this structure reduces the size of SG and non-SG RX queue elements
significantly to improve performance.

An nice side effect is that the mbuf pointer is now fully stored in
struct rxq_elt instead of relying on the WR ID data offset hack.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Signed-off-by: Olga Shern <olgas@mellanox.com>
Signed-off-by: Or Ami <ora@mellanox.com>
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>