vfio/iommufd: Fix SIGSEV in iommufd_cdev_attach()

vfio/iommufd: Fix SIGSEV in iommufd_cdev_attach()

When iommufd_cdev_ram_block_discard_disable() fails for whatever reason,
errp should be set or else SIGSEV is triggered in vfio_realize() when
error_prepend() is called.

By this chance, use the same error message for both legacy and iommufd
backend.

Fixes: 5ee3dc7af785 ("vfio/iommufd: Implement the iommufd backend")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20250116102307.260849-1-zhenzhong.duan@intel.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>