Handle UMA_ANYDOMAIN in kstack_import().

The kernel thread stack zone performs first-touch allocations by
default, and must handle the case where the local memory domain
is empty. For most UMA zones this is handled in the keg layer,
but cache zones currently must implement a policy for this case.
Simply use a round-robin policy if UMA_ANYDOMAIN is passed.

Reported and tested by: bcran
Reviewed by: kib
Sponsored by: The FreeBSD Foundation