md: Fix raid1->raid0 takeover

Takeover raid1->raid0 not succeded. Kernel message is shown:
"md/raid0:md126: too few disks (1 of 2) - aborting!"

Problem was that we weren't updating ->raid_disks for that
takeover, unlike all the others.
Signed-off-by: NKrzysztof Wojcik <krzysztof.wojcik@intel.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid0.c

@@ -670,6 +670,7 @@ static void *raid0_takeover_raid1(mddev_t *mddev)
 	mddev->new_layout = 0;
 	mddev->new_chunk_sectors = 128; /* by default set chunk size to 64k */
 	mddev->delta_disks = 1 - mddev->raid_disks;
+	mddev->raid_disks = 1;
 	/* make sure it will be not marked as dirty */
 	mddev->recovery_cp = MaxSector;