cifs.spnego key descriptions contain authority-bearing fields such as
    pid, uid, creduid, and upcall_target that cifs.upcall treats as
    kernel-originating inputs. However, userspace can also create keys of
    this type through request_key(2) or add_key(2), allowing those fields to
    be supplied without CIFS origin.
    Only accept cifs.spnego descriptions while CIFS is using its private
    spnego_cred to request the key.