md: ensure all blocks are uptodate or locked when syncing

Remove the dubious attempt to prefer 'compute' over 'read'.  Not only is it
wrong given commit c337869d (md: do not compute parity unless it is on a failed
drive), but it can trigger a BUG_ON in handle_parity_checks5().

Cc: <stable@kernel.org>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NNeil Brown <neilb@suse.de>

drivers/md/raid5.c

@@ -2017,12 +2017,7 @@ static int __handle_issuing_new_read_requests5(struct stripe_head *sh,
 			 */
 			s->uptodate++;
 			return 0; /* uptodate + compute == disks */
-		} else if ((s->uptodate < disks - 1) &&
-			test_bit(R5_Insync, &dev->flags)) {
-			/* Note: we hold off compute operations while checks are
-			 * in flight, but we still prefer 'compute' over 'read'
-			 * hence we only read if (uptodate < * disks-1)
-			 */
+		} else if (test_bit(R5_Insync, &dev->flags)) {
 			set_bit(R5_LOCKED, &dev->flags);
 			set_bit(R5_Wantread, &dev->flags);
 			if (!test_and_set_bit(STRIPE_OP_IO, &sh->ops.pending))