QAT: Allocate digest_buffer using QAT_PHYS_CONTIG_ALLOC()

If the buffer 'digest_buffer' is allocated in the qat_checksum()
stack, it can't ensure that the address is physically contiguous,
and the DMA result of the buffer may be handled incorrectly.
Using QAT_PHYS_CONTIG_ALLOC() ensures a physically
contiguous allocation.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Chengfei, Zhu <chengfeix.zhu@intel.com>
Closes #8323
Closes #8521