Btrfs: Count async bios separately from async checksum work items

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index c88f1e1..bb4a8d2 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -544,6 +544,7 @@
 	struct list_head hashers;
 	struct list_head dead_roots;
 	atomic_t nr_async_submits;
+	atomic_t nr_async_bios;
 
 	/*
 	 * this is used by the balancing code to wait for all the pending