Btrfs: save us a read_lock

This does not change the logic of code, but can save us a read_lock.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/locking.c

@@ -113,11 +113,10 @@ void btrfs_tree_read_lock(struct extent_buffer *eb)
 		read_unlock(&eb->lock);
 		return;
 	}
-	read_unlock(&eb->lock);
-	wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
-	read_lock(&eb->lock);
 	if (atomic_read(&eb->blocking_writers)) {
 		read_unlock(&eb->lock);
+		wait_event(eb->write_lock_wq,
+			   atomic_read(&eb->blocking_writers) == 0);
 		goto again;
 	}
 	atomic_inc(&eb->read_locks);