Use nitems to get the correct number of registers to read when dumping

them. Previously this would walk past the end of the array and print
whatever happened to be after the trapframe struct.

MFC after:	1 week
Sponsored by:	DARPA, AFRL

sys/arm64/arm64/trap.c

@@ -250,7 +250,7 @@ print_registers(struct trapframe *frame)
 {
 	u_int reg;
 
-	for (reg = 0; reg < 31; reg++) {
+	for (reg = 0; reg < nitems(frame->tf_x); reg++) {
 		printf(" %sx%d: %16lx\n", (reg < 10) ? " " : "", reg,
 		    frame->tf_x[reg]);
 	}