Change U16 to U32 due to atomic_inc_32_nv

After run a long time with QAT compression, the variable "inst_num"
is overflow by "atomic_inc_32_nv", which causes its neighbor
variable overwritten. Change its definition from U16 to U32.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Weigang Li <weigang.li@intel.com>
Closes #6051