RDMA/mlx5: Use flags_en mechanism to mark QP created with WQE signature

RDMA/mlx5: Use flags_en mechanism to mark QP created with WQE signature

MLX5_QP_FLAG_SIGNATURE is exposed to the users but in the kernel
the create_qp flow treated it differently from other MLX5_QP_FLAG_*s.
Fix it by ditching wq_sig boolean variable and use general flag_en
mechanism.

Link: https://lore.kernel.org/r/20200427154636.381474-16-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>