Truncating the quota files can silently fail. Ensure that truncation
errors are propagated to the callers.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30791a
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>