treat C_SCSIXFER as a reference count and use it as a guard to pool_put.
treat C_SCSIXFER as a reference count and use it as a guard to pool_put.

fixes cd panics where the free_xfer path is called after scsi_done calls
xfer_put.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars