cdrom: replace hard-coded constants by kernel.h macro.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>