md: Cleanup after raid45->raid0 takeover

Problem:
After raid4->raid0 takeover operation, another takeover operation
(e.g raid0->raid10) results "kernel oops".
Root cause:
Variables 'degraded' in mddev structure is not cleared
on raid45->raid0 takeover.

This patch reset this variable.
Signed-off-by: NKrzysztof Wojcik <krzysztof.wojcik@intel.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -3170,6 +3170,7 @@ level_store(mddev_t *mddev, const char *buf, size_t len)
 	mddev->layout = mddev->new_layout;
 	mddev->chunk_sectors = mddev->new_chunk_sectors;
 	mddev->delta_disks = 0;
+	mddev->degraded = 0;
 	if (mddev->pers->sync_request == NULL) {
 		/* this is now an array without redundancy, so
 		 * it must always be in_sync