Fix coverity defects: CID 150926

CID 150926: Unchecked return value (CHECKED_RETURN)

• This case cannot occur given the existing taskq implementation and flags passed to task_dispatch().

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: luozhengzheng <luo.zhengzheng@zte.com.cn>
Closes #5272