xfs: bulletfproof xfs_qm_scall_trunc_qfiles()

Coverity noticed that if we sent junk into
xfs_qm_scall_trunc_qfiles(), we could get back an
uninitialized error value.  So sanitize the flags we
will accept, and initialize error anyway for good measure.

(This bug may have been introduced via c61a9e39

).

Should resolve Coverity CID 1163872.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>