md/raid5: Ensure a batch member is not handled prematurely.

If a stripe is a member of a batch, but not the head, it must
not be handled separately from the rest of the batch.

'clear_batch_ready()' handles this requirement to some
extent but not completely.  If a member is passed to handle_stripe()
a second time it returns '0' indicating the stripe can be handled,
which is wrong.
So add an extra test.
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -4200,9 +4200,13 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
 
 static int clear_batch_ready(struct stripe_head *sh)
 {
+	/* Return '1' if this is a member of batch, or
+	 * '0' if it is a lone stripe or a head which can now be
+	 * handled.
+	 */
 	struct stripe_head *tmp;
 	if (!test_and_clear_bit(STRIPE_BATCH_READY, &sh->state))
-		return 0;
+		return (sh->batch_head && sh->batch_head != sh);
 	spin_lock(&sh->stripe_lock);
 	if (!sh->batch_head) {
 		spin_unlock(&sh->stripe_lock);