Add GET_STACK_USAGE() for arm64.

Its absence meant that GEOM direct dispatch was disabled (the service
routines check the current thread's stack usage to determine whether
to hand off the request to a dedicated thread), and this change is
sufficient to enable direct dispatch by default.

Reviewed by:	allanjude
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D15527

sys/arm64/include/proc.h

@@ -54,4 +54,18 @@ struct syscall_args {
 	int narg;
 };
 
+#ifdef _KERNEL
+
+#include <machine/pcb.h>
+
+#define	GET_STACK_USAGE(total, used) do {				\
+	struct thread *td = curthread;					\
+	(total) = td->td_kstack_pages * PAGE_SIZE - sizeof(struct pcb);	\
+	(used) = (char *)td->td_kstack +				\
+	    td->td_kstack_pages * PAGE_SIZE -				\
+	    (char *)&td;						\
+} while (0)
+
+#endif
+
 #endif /* !_MACHINE_PROC_H_ */