md: rename mddev->write_lock to mddev->lock

This lock is used for (slightly) more than helping with writing
superblocks, and it will soon be extended further.  So the
name is inappropriate.

Also, the _irq variant hasn't been needed since 2.6.37 as it is
never taking from interrupt or bh context.

So:
  -rename write_lock to lock
  -document what it protects
  -remove _irq ... except in md_flush_request() as there
     is no wait_event_lock() (with no _irq).  This can be
     cleaned up after appropriate changes to wait.h.

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

drivers/md/md.h

@@ -386,7 +386,13 @@ struct mddev {
 
 	struct work_struct del_work;	/* used for delayed sysfs removal */
 
-	spinlock_t			write_lock;
+	/* "lock" protects:
+	 *   flush_bio transition from NULL to !NULL
+	 *   rdev superblocks, events
+	 *   clearing MD_CHANGE_*
+	 *   in_sync - and related safemode and MD_CHANGE changes
+	 */
+	spinlock_t			lock;
 	wait_queue_head_t		sb_wait;	/* for waiting on superblock updates */
 	atomic_t			pending_writes;	/* number of active superblock writes */