kmem_zalloc with KM_SLEEP will never return NULL

These allocations can never fail. Leaving the error handling
code here gives the impression they can so it has been removed.

Signed-off-by: luozhengzheng <luo.zhengzheng@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5048