iommufd: Fix return value of iommufd_fault_fops_write()

iommufd: Fix return value of iommufd_fault_fops_write()

copy_from_user() may return number of bytes failed to copy, we should
not pass over this number to user space to cheat that write() succeed.
Instead, -EFAULT should be returned.

Link: https://patch.msgid.link/r/20260330030755.12856-1-zhenzhong.duan@intel.com
Cc: stable@vger.kernel.org
Fixes: 07838f7fd529 ("iommufd: Add iommufd fault object")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Reviewed-by: Shuai Xue <xueshuai@linux.alibaba.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>