Cleanup: Use kmem_zalloc() instead of memset() to zero memory

The Linux 5.16.14 kernel's coccicheck caught this. The semantic patch
that caught it was:

./scripts/coccinelle/api/alloc/zalloc-simple.cocci

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14372