vm: Fix error handling in vm_thread_stack_back()

vm_object_page_remove() wants to busy the page, but that won't work
here. (Kernel stack pages are always busy.)

Make the error handling path look more like vm_thread_stack_dispose().

Reported by: pho
Reviewed by: kib, bnovkov
Fixes: 7a79d0669761 ("vm: improve kstack_object pindex calculation to avoid pindex holes")
Differential Revision: https://reviews.freebsd.org/D45019