SYSDB: Sanitize filter before removing ghost attrs
    
    sysdb_add_user fails with EIO if enumeration is disabled and user contains
    backslashes.
    We try to remove ghost attributes from groups with disabled enumeration,
    but unsanitized filter is used to find ghost attributes
    "(|(ghost=usr\\\\002)" and ldb cannot parse this filter.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2163