btrfs: add define for oldest generation

Some functions can filter metadata by the generation. Add a define that
will annotate such arguments.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index f4926dc..15bf111 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -66,6 +66,8 @@ struct btrfs_ordered_sum;
 
 #define BTRFS_MAX_LEVEL 8
 
+#define BTRFS_OLDEST_GENERATION	0ULL
+
 #define BTRFS_COMPAT_EXTENT_TREE_V0
 
 /*