IB/core: Better checking of userspace values for receive flow steering

  - Don't allow unsupported comp_mask values, user should check
    ibv_query_device to know which features are supported.
  - Add a check in ib_uverbs_create_flow() to verify the size passed
    from the user space.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>