Fix the location of td->td_frame at the top of the kernel stack.

In cpu_thread_alloc we would allocate space for the trap frame at the top of
the kernel stack. This is just below the pcb, however due to a missing cast
the pointer arithmetic would use the pcb size, not the trapframe size. As
the pcb is larger than the trapframe this is safe, however later in cpu_fork
we include the case leading to the two disagreeing on the location.

Fix by using the same arithmetic in both locations.

Found by:	An early KASAN patch
Sponsored by:	DARPA, AFRL

sys/arm64/arm64/vm_machdep.c

@@ -228,7 +228,7 @@ cpu_thread_alloc(struct thread *td)
 	td->td_pcb = (struct pcb *)(td->td_kstack +
 	    td->td_kstack_pages * PAGE_SIZE) - 1;
 	td->td_frame = (struct trapframe *)STACKALIGN(
-	    td->td_pcb - 1);
+	    (struct trapframe *)td->td_pcb - 1);
 }
 
 void