Optimize xpt_getattr().

Do not allocate temporary buffer for attributes we are going to return
as-is, just make sure to NUL-terminate them. Do not zero temporary 64KB
buffer for CDAI_TYPE_SCSI_DEVID, XPT tells us how much data it filled
and there are also length fields inside the returned data also.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.