md/bitmap: protect clearing of ->bitmap by mddev->lock

This makes it safe to inspect the struct while holding only
the spinlock.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.h

@@ -392,6 +392,7 @@ struct mddev {
 	 *   clearing MD_CHANGE_*
 	 *   in_sync - and related safemode and MD_CHANGE changes
 	 *   pers (also protected by reconfig_mutex and pending IO).
+	 *   clearing ->bitmap
 	 */
 	spinlock_t			lock;
 	wait_queue_head_t		sb_wait;	/* for waiting on superblock updates */