Btrfs: create a logical->phsyical block number mapping scheme

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 01310de..454eb88 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -242,6 +242,7 @@
 	struct radix_tree_root fs_roots_radix;
 	struct radix_tree_root pending_del_radix;
 	struct radix_tree_root pinned_radix;
+	struct radix_tree_root dev_radix;
 	u64 generation;
 	struct btrfs_transaction *running_transaction;
 	struct btrfs_super_block *disk_super;