md: Fix unfortunate interaction with evms

evms configures md arrays by:
  open device
  send ioctl
  close device

for each different ioctl needed.
Since 2.6.29, the device can disappear after the 'close'
unless a significant configuration has happened to the device.
The change made by "SET_ARRAY_INFO" can too minor to stop the device
from disappearing, but important enough that losing the change is bad.

So: make sure SET_ARRAY_INFO sets mddev->ctime, and keep the device
active as long as ctime is non-zero (it gets zeroed with lots of other
things when the array is stopped).

This is suitable for -stable kernels since 2.6.29.
Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org

drivers/md/md.c

@@ -386,7 +386,9 @@ static void mddev_put(mddev_t *mddev)
 	if (!atomic_dec_and_lock(&mddev->active, &all_mddevs_lock))
 		return;
 	if (!mddev->raid_disks && list_empty(&mddev->disks) &&
-	    !mddev->hold_active) {
+	    mddev->ctime == 0 && !mddev->hold_active) {
+		/* Array is not configured at all, and not held active,
+		 * so destroy it */
 		list_del(&mddev->all_mddevs);
 		if (mddev->gendisk) {
 			/* we did a probe so need to clean up.
@@ -5262,6 +5264,10 @@ static int set_array_info(mddev_t * mddev, mdu_array_info_t *info)
 		mddev->minor_version = info->minor_version;
 		mddev->patch_version = info->patch_version;
 		mddev->persistent = !info->not_persistent;
+		/* ensure mddev_put doesn't delete this now that there
+		 * is some minimal configuration.
+		 */
+		mddev->ctime         = get_seconds();
 		return 0;
 	}
 	mddev->major_version = MD_MAJOR_VERSION;