Only check modrdn ops for backend/suffix correctness if not the default backend
    
    The function slapi_mapping_tree_select_and_check() is only called for
    modrdn operations, to make sure we are not attempting to rename a suffix
    or move an entry from one backend to another.  This defeats datainterop
    plugins that may want to perform some other operation in these cases.  If
    the target suffix/backend is not found, the default backend is used.  If
    the default backend is being used, don't check for all errors, just allow
    the operation to pass through to the preop plugins.
    Need to make sure this doesn't cause problems if
    1) null suffix is not used - entry really is bogus or doesn't exist
    2) null suffix is being used but entry belongs to another null suffix or
    is really bogus
    Reviewed by: nhosoi (Thanks!)
    Tested on: Fedora 14 x86_64