Cleanup: Remove unnecessary explicit casts of pointers from allocators

The Linux 5.16.14 kernel's coccicheck caught these. The semantic patch
that caught them was:

./scripts/coccinelle/api/alloc/alloc_cast.cocci

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