RDMA/mlx5: Rely on existence of udata to separate kernel/user flows

Instead of keeping special field to separate kernel/user create/destroy
flows, rely on existence of udata pointer. All allocation flows are
using kzalloc() and leave uninitialized pointers as NULL which makes
MLX5_QP_EMPTY and MLX5_QP_KERNEL flows to be the same.

Link: https://lore.kernel.org/r/20200427154636.381474-25-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>