mkfs.btrfs: return some defined value instead of garbage when lookup checksum

mkfs.btrfs: return some defined value instead of garbage when lookup checksum

==31873== Command: ./mkfs.btrfs -r /some/root/
==31873== Parent PID: 31872
==31873==
==31873== Conditional jump or move depends on uninitialised value(s)
==31873==    at 0x42C3D0: add_file_items (mkfs.c:792)
==31873==    by 0x42CAB3: traverse_directory (mkfs.c:948)
==31873==    by 0x42CF11: make_image (mkfs.c:1047)
==31873==    by 0x42DE53: main (mkfs.c:1401)
==31873==  Uninitialised value was created by a stack allocation
==31873==    at 0x41B1B1: btrfs_csum_file_block (file-item.c:195)

'ret' value was not initialized for 'found' branch.

The same fix sits in kernel:
> commit 639cb58675ce9b507eed9c3d6b3335488079b21a
> Author: Chris Mason <chris.mason@oracle.com>
> Date:   Thu Aug 28 06:15:25 2008 -0400
>
>     Btrfs: Fix variable init during csum creation
>
>     Signed-off-by: Chris Mason <chris.mason@oracle.com>

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Hugo Mills <hugo@carfax.org.uk>