Cleanup: Use NULL when doing NULL pointer comparisons

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

./scripts/coccinelle/null/badzero.cocci

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