The most common way to set ECE option will be during modify QP command in
INIT2RTR, RTR2RTS and RTS2RTS stages, so update mlx5 to support it.

The new bit in the comp_mask is needed to mark that kernel supports ECE
and can receive data instead of "reserved" field in the struct
mlx5_ib_modify_qp.

Link: https://lore.kernel.org/r/20200526115440.205922-8-leon@kernel.orgReviewed-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>