Add option to disable multipath and other fixes

Signed-off-by: Utkarsh Mani Tripathi <utkarsh.tripathi@mayadata.io>

pkg/scsi/backingstore.go

@@ -125,12 +125,12 @@ func bsPerformCommand(bs api.BackingStore, cmd *api.SCSICommand) (err error, key
 			bs.DataAdvise(int64(offset), int64(length), util.POSIX_FADV_NOREUSE)
 		}
 		cmd.InSDBBuffer.Resid = uint32(length)
-		copy(cmd.InSDBBuffer.Buffer, rbuf)
 		if cmd.InSDBBuffer.Length < uint32(length) {
 			key = ILLEGAL_REQUEST
 			asc = ASC_INVALID_FIELD_IN_CDB
 			goto sense
 		}
+		copy(cmd.InSDBBuffer.Buffer, rbuf)
 	case api.PRE_FETCH_10, api.PRE_FETCH_16:
 		err = bs.DataAdvise(int64(offset), tl, util.POSIX_FADV_WILLNEED)
 		if err != nil {