Btrfs: Fold some btree readahead routines into something more generic.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 73c2e75..c5a18d5 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -177,6 +177,7 @@
 struct btrfs_path {
 	struct buffer_head *nodes[BTRFS_MAX_LEVEL];
 	int slots[BTRFS_MAX_LEVEL];
+	int reada;
 };
 
 /*