crypto: qat - remove dma_free_coherent() for DH

The functions qat_dh_compute_value() allocates memory with
dma_alloc_coherent() if the source or the destination buffers are made
of multiple flat buffers or of a size that is not compatible with the
hardware.
This memory is then freed with dma_free_coherent() in the context of a
tasklet invoked to handle the response for the corresponding request.

According to Documentation/core-api/dma-api-howto.rst, the function
dma_free_coherent() cannot be called in an interrupt context.

Replace allocations with dma_alloc_coherent() in the function
qat_dh_compute_value() with kmalloc() + dma_map_single().

Cc: stable@vger.kernel.org
Fixes: c9839143 ("crypto: qat - Add DH support")
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Adam Guerin <adam.guerin@intel.com>
Reviewed-by: Wojciech Ziemba <wojciech.ziemba@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>