Fix inst_num overflow in qat_crypt.c

This patch fixes the same issue which was previously addressed in

6051. The variable "inst_num" was of the incorrect type and

"atomic_inc_32_nv()" could cause an overflow damaging its neighbor.

Cast the return value of atomic_inc_32_nv() to Cpa32U.

Fix a few types for num_inst for clarity.

Reviewed-by: Weigang Li <weigang.li@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7468