md: factor out a helper rdev_is_spare() from remove_and_add_spares()

There are no functional changes, just to make the code simpler and
prepare to delay remove_and_add_spares() to md_start_sync().
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230825031622.1530464-6-yukuai1@huaweicloud.com

drivers/md/md.c

@@ -9200,6 +9200,14 @@ static bool rdev_removeable(struct md_rdev *rdev)
 	return false;
 }
 
+static bool rdev_is_spare(struct md_rdev *rdev)
+{
+	return !test_bit(Candidate, &rdev->flags) && rdev->raid_disk >= 0 &&
+	       !test_bit(In_sync, &rdev->flags) &&
+	       !test_bit(Journal, &rdev->flags) &&
+	       !test_bit(Faulty, &rdev->flags);
+}
+
 static int remove_and_add_spares(struct mddev *mddev,
 				 struct md_rdev *this)
 {
@@ -9255,13 +9263,10 @@ static int remove_and_add_spares(struct mddev *mddev,
 	rdev_for_each(rdev, mddev) {
 		if (this && this != rdev)
 			continue;
+		if (rdev_is_spare(rdev))
+			spares++;
 		if (test_bit(Candidate, &rdev->flags))
 			continue;
-		if (rdev->raid_disk >= 0 &&
-		    !test_bit(In_sync, &rdev->flags) &&
-		    !test_bit(Journal, &rdev->flags) &&
-		    !test_bit(Faulty, &rdev->flags))
-			spares++;
 		if (rdev->raid_disk >= 0)
 			continue;
 		if (test_bit(Faulty, &rdev->flags))