target: Return correct ASC for unimplemented VPD pages

My draft of SPC-4 says:

    If the device server does not implement the requested vital product
    data page, then the command shall be terminated with CHECK CONDITION
    status, with the sense key set to ILLEGAL REQUEST, and the
    additional sense code set to INVALID FIELD IN CDB.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>