xfs: don't unconditionally clear the reflink flag on zero-block files

If we have speculative cow preallocations hanging around in the cow
fork, don't let a truncate operation clear the reflink flag because if
we do then there's a chance we'll forget to free those extents when we
destroy the incore inode.

Reported-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>