scsi_debug: use likely hints on fast path

The most common commands in normal use are the READ and WRITE SCSI
commands. Use likely and unlikely hints along the path taken by these
commands. Rename check_readiness() to make_ua() and remove associated
dead code. Rename devInfoReg() to find_build_dev_info().

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>