MD: raid5 do not set fullsync

Add check to determine if a device needs full resync or if partial resync will do

RAID 5 was assuming that if a device was not In_sync, it must undergo a full
resync.  We add a check to see if 'saved_raid_disk' is the same as 'raid_disk'.
If it is, we can safely skip the full resync and rely on the bitmap for
partial recovery instead.  This is the legitimate purpose of 'saved_raid_disk',
from md.h:
int saved_raid_disk;            /* role that device used to have in the
                                 * array and could again if we did a partial
                                 * resync from the bitmap
                                 */
Signed-off-by: NJonathan Brassow <jbrassow@redhat.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -4858,7 +4858,7 @@ static raid5_conf_t *setup_conf(mddev_t *mddev)
 			printk(KERN_INFO "md/raid:%s: device %s operational as raid"
 			       " disk %d\n",
 			       mdname(mddev), bdevname(rdev->bdev, b), raid_disk);
-		} else
+		} else if (rdev->saved_raid_disk != raid_disk)
 			/* Cannot rely on bitmap to complete recovery */
 			conf->fullsync = 1;
 	}