cups/backend smp normally points to smb -> ../../../bin/smbspool but in order to use Kerberos to authenticate one self this symlink needs to point to sambas smbspool_krb5_wrapper: smb -> ../../samba/smbspool_krb5_wrapper and there is no way to set this up so the sym link survives an cups rebuild/upgrade.
This symlink is actually installed by samba so this bug does not belong to cups, closing.