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

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 <[email protected]>
Reviewed-by: Carlos Maiolino <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>