The expectation that the allocation will be aligned to the kstack size
only applies when allocating from a kstack arena, not when allocating a
non-standard size from the kernel arena.
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Fixes: 7a79d0669761 ("vm: improve kstack_object pindex calculation to avoid pindex holes")