udev/cdrom_id: Do not open CD-rom in exclusive mode.

udev/cdrom_id: Do not open CD-rom in exclusive mode.

When you have a CD automunt solution that talks directly to the kernel
independently of udev it races with cdrom_id for exclusive access to the
device failing unpredictably.

The whole is_mounted function in cdrom_id is broken: there is no saying
what happens between calling is_mounted and opening the device.

Hence assume that the device can be mounted asynchronously at any time,
do not use exclusive access, and do away with is_mouted.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>