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/ioctl.c b/fs/btrfs/ioctl.c
index c5a5591..ba403c0 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2600,7 +2600,7 @@ static int btrfs_ioctl_defrag(struct file *file, void __user *argp)
 			range->len = (u64)-1;
 		}
 		ret = btrfs_defrag_file(file_inode(file), file,
-					range, 0, 0);
+					range, BTRFS_OLDEST_GENERATION, 0);
 		if (ret > 0)
 			ret = 0;
 		kfree(range);