gdrom: fix a memory leak bug

In probe_gdrom(), the buffer pointed by 'gd.cd_info' is allocated through
kzalloc() and is used to hold the information of the gdrom device. To
register and unregister the device, the pointer 'gd.cd_info' is passed to
the functions register_cdrom() and unregister_cdrom(), respectively.
However, this buffer is not freed after it is used, which can cause a
memory leak bug.

This patch simply frees the buffer 'gd.cd_info' in exit_gdrom() to fix the
above issue.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>