Btrfs: keep irqs on more often in the worker threads

The btrfs worker thread spinlock was being used both for the
queueing of IO and for the processing of ordered events.

The ordered events never happen from end_io handlers, and so they
don't need to use the _irq version of spinlocks.  This adds a
dedicated lock to the ordered lists so they don't have to run
with irqs off.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/async-thread.h

@@ -99,6 +99,9 @@ struct btrfs_workers {
 	/* lock for finding the next worker thread to queue on */
 	spinlock_t lock;
 
+	/* lock for the ordered lists */
+	spinlock_t order_lock;
+
 	/* extra name for this worker, used for current->name */
 	char *name;
 };