md/raid5: Fix Force reconstruct-write io stuck in degraded raid5

In degraded raid5, we need to read parity to do reconstruct-write when
data disks fail. However, we can not read parity from
handle_stripe_dirtying() in force reconstruct-write mode.

Reproducible Steps:

1. Create degraded raid5
mdadm -C /dev/md2 --assume-clean -l5 -n3 /dev/sda2 /dev/sdb2 missing
2. Set rmw_level to 0
echo 0 > /sys/block/md2/md/rmw_level
3. IO to raid5

Now some io may be stuck in raid5. We can use handle_stripe_fill() to read
the parity in this situation.

Cc: <stable@vger.kernel.org> # v4.4+
Reviewed-by: Alex Wu <alexwu@synology.com>
Reviewed-by: BingJing Chang <bingjingc@synology.com>
Reviewed-by: Danny Shih <dannyshih@synology.com>
Signed-off-by: ChangSyun Peng <allenpeng@synology.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

Author: ChangSyun Peng

drivers/md/raid5.c

@@ -3626,6 +3626,7 @@ static int need_this_block(struct stripe_head *sh, struct stripe_head_state *s,
 	 * is missing/faulty, then we need to read everything we can.
 	 */
 	if (sh->raid_conf->level != 6 &&
+	    sh->raid_conf->rmw_level != PARITY_DISABLE_RMW &&
 	    sh->sector < sh->raid_conf->mddev->recovery_cp)
 		/* reconstruct-write isn't being forced */
 		return 0;
@@ -4862,7 +4863,7 @@ static void handle_stripe(struct stripe_head *sh)
 	 * or to load a block that is being partially written.
 	 */
 	if (s.to_read || s.non_overwrite
-	    || (conf->level == 6 && s.to_write && s.failed)
+	    || (s.to_write && s.failed)
 	    || (s.syncing && (s.uptodate + s.compute < disks))
 	    || s.replacing
 	    || s.expanding)