After commit 7a79d0669761 we no longer hold the VM object lock when
initializing or advancing the domainset iterator in
vm_thread_stack_create().

We could lift the lock out of vm_thread_stack_back() but that poses
complications since we cannot allocate KVA with a VM object lock held.

Instead of overridding the object's iterator, just borrow that of the
current thread. Kernel stacks are basically always allocated with a
DOMAINSET_POLICY_PREFER policy, so it's not very important to maintain a
global iterator for round-robin allocation.

As a part of this, fix up flag handling: make sure we handle
M_NOWAIT/M_WAITOK from the caller. Delete a comment in vm_thread_new()
which refers to a non-existent consideration (stack swapping has been
removed). I suspect vm_thread_new() can use M_WAITOK but opted not to
make that change here.

Fixes: 7a79d0669761 ("vm: improve kstack_object pindex calculation to avoid pindex holes")