btrfs: scrub: silence an uninitialized variable warning

It's basically harmless if "ref_level" isn't initialized since it's only
used for an error message, but it causes a static checker warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>