md/raid6: eliminate BUG_ON with side effect

As pointed out by Neil it should be possible to build a driver with all
BUG_ON statements deleted.  It's bad form to have a BUG_ON with a side
effect.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 9b00a22..0a5f03d 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3214,8 +3214,10 @@
 		/* now count some things */
 		if (test_bit(R5_LOCKED, &dev->flags)) s.locked++;
 		if (test_bit(R5_UPTODATE, &dev->flags)) s.uptodate++;
-		if (test_bit(R5_Wantcompute, &dev->flags))
-			BUG_ON(++s.compute > 2);
+		if (test_bit(R5_Wantcompute, &dev->flags)) {
+			s.compute++;
+			BUG_ON(s.compute > 2);
+		}
 
 		if (test_bit(R5_Wantfill, &dev->flags)) {
 			s.to_fill++;