MFC r269631:
Reduce reported additional INQUIRY data length.

sizeof(struct scsi_inquiry_data) of 256 bytes combined with off-by-one
error in the changed code gave total INQUIRY data length above 255 bytes,
that was maximal INQUIRY length in SPC-2. While SPC-3 increased the
maximal length to 64K, at least sg3_utils are still confused by that.