Avoid uninitialized variant compile warning

Avoid uninitialized variant compile warning

When we compile btrfs-progs in RHEL5(with default gcc version 4.1.2 20070626),
we can get following error:

cc1: warnings being treated as errors
btrfs-list.c: In function 'find_updated_files':
btrfs-list.c:668: warning: 'disk_offset' may be used uninitialized in this function
btrfs-list.c:667: warning: 'disk_start' may be used uninitialized in this function
btrfs-list.c:666: warning: 'len' may be used uninitialized in this function
make: *** [btrfs-list.o] Error 1

These varient are always initialized except inconsistent data in file system.
We can set initial value for these variant for this situation.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>