Use vmem_zalloc to silence allocation warning

The kmem allocation in zfs_prune_aliases() will trigger a large
allocation warning on systems with 64K pages. Resolve this by
switching to vmem_alloc() which internally uses kvmalloc() so the
right allocator will be used based on the allocation size.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8491
Closes #14694