ARM: unwinder: fix bisection to find origin in .idx section

The bisection implemented in unwind_find_origin() stopped to early.  If
there is only a single entry left to check the original code just took
the end point as origin which might be wrong.

This was introduced in commit de66a979012d ("ARM: 7187/1: fix unwinding
for XIP kernels").

Reported-and-tested-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm/kernel/unwind.c

@@ -160,12 +160,12 @@ static const struct unwind_idx *unwind_find_origin(
 		const struct unwind_idx *start, const struct unwind_idx *stop)
 {
 	pr_debug("%s(%p, %p)\n", __func__, start, stop);
-	while (start < stop - 1) {
+	while (start < stop) {
 		const struct unwind_idx *mid = start + ((stop - start) >> 1);
 
 		if (mid->addr_offset >= 0x40000000)
 			/* negative offset */
-			start = mid;
+			start = mid + 1;
 		else
 			/* positive offset */
 			stop = mid;