CIFS: fiemap: do not return EINVAL if get nothing

If we call fiemap on a truncated file with none blocks allocated,
it makes sense we get nothing from this call. No output means
no blocks have been counted, but the call succeeded. It's a valid
response.

Simple example reproducer:
xfs_io -f 'truncate 2M' -c 'fiemap -v' /cifssch/testfile
xfs_io: ioctl(FS_IOC_FIEMAP) ["/cifssch/testfile"]: Invalid argument

Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/smb2ops.c

@@ -3419,7 +3419,7 @@ static int smb3_fiemap(struct cifs_tcon *tcon,
 	if (rc)
 		goto out;
 
-	if (out_data_len < sizeof(struct file_allocated_range_buffer)) {
+	if (out_data_len && out_data_len < sizeof(struct file_allocated_range_buffer)) {
 		rc = -EINVAL;
 		goto out;
 	}