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>

fs/xfs/xfs_inode.c

@@ -1624,10 +1624,12 @@ xfs_itruncate_extents(
 		goto out;
 
 	/*
-	 * Clear the reflink flag if we truncated everything.
+	 * Clear the reflink flag if there are no data fork blocks and
+	 * there are no extents staged in the cow fork.
 	 */
-	if (ip->i_d.di_nblocks == 0 && xfs_is_reflink_inode(ip)) {
-		ip->i_d.di_flags2 &= ~XFS_DIFLAG2_REFLINK;
+	if (xfs_is_reflink_inode(ip) && ip->i_cnextents == 0) {
+		if (ip->i_d.di_nblocks == 0)
+			ip->i_d.di_flags2 &= ~XFS_DIFLAG2_REFLINK;
 		xfs_inode_clear_cowblocks_tag(ip);
 	}