drivers/cdrom/gdrom.c: fix device number leak

Without this patch, gdrom_major will leak when gd.cd_info alloc fails.

Signed-off-by: Libo Chen <libo.chen@huawei.com>
Cc: Jens Axboe <axboe@kernel.dk>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>