UBIFS: fix zero-length truncations

Always allow truncations to zero, even if budgeting thinks there
is no space. UBIFS reserves some space for deletions anyway.

Otherwise, the following happans:
1. create a file, and write as much as possible there, until ENOSPC
2. truncate the file, which fails with ENOSPC, which is not good.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>