gntdev: fix multi-page slot allocation

Any range with the first slot available would have got considered
usable, as range_length never got reset when encountering an in-use
slot.

Additionally fold the two almost identical loops into a single
instance, at once avoiding to go through both loops when start_index
was zero even for the first one.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/gntdev/gntdev.c
+++ b/drivers/xen/gntdev/gntdev.c
@@ -285,35 +285,27 @@ static void compress_free_list(gntdev_fi
 static int find_contiguous_free_range(gntdev_file_private_data_t *private_data,
 				      uint32_t num_slots) 
 {
-	uint32_t i, start_index = private_data->next_fit_index;
-	uint32_t range_start = 0, range_length;
-
-	/* First search from the start_index to the end of the array. */
-	range_length = 0;
-	for (i = start_index; i < private_data->grants_size; ++i) {
-		if (private_data->grants[i].state == GNTDEV_SLOT_INVALID) {
-			if (range_length == 0) {
-				range_start = i;
-			}
-			++range_length;
-			if (range_length == num_slots) {
-				return range_start;
-			}
-		}
-	}
-	
-	/* Now search from the start of the array to the start_index. */
-	range_length = 0;
-	for (i = 0; i < start_index; ++i) {
-		if (private_data->grants[i].state == GNTDEV_SLOT_INVALID) {
-			if (range_length == 0) {
-				range_start = i;
-			}
-			++range_length;
-			if (range_length == num_slots) {
-				return range_start;
-			}
-		}
+	/* First search from next_fit_index to the end of the array. */
+	uint32_t start_index = private_data->next_fit_index;
+	uint32_t end_index = private_data->grants_size;
+
+	for (;;) {
+		uint32_t i, range_start = 0, range_length = 0;
+
+		for (i = start_index; i < end_index; ++i) {
+			if (private_data->grants[i].state == GNTDEV_SLOT_INVALID) {
+				if (range_length == 0)
+					range_start = i;
+				if (++range_length == num_slots)
+					return range_start;
+			} else
+				range_length = 0;
+		}
+		/* Now search from the start of the array to next_fit_index. */
+		if (!start_index)
+			break;
+		end_index = start_index;
+		start_index = 0;
 	}
 	
 	return -ENOMEM;