ublk_drv: add mechanism for supporting unprivileged ublk device

ublk_drv: add mechanism for supporting unprivileged ublk device

unprivileged ublk device is helpful for container use case, such
as: ublk device created in one unprivileged container can be controlled
and accessed by this container only.

Implement this feature by adding flag of UBLK_F_UNPRIVILEGED_DEV, and if
this flag isn't set, any control command has been run from privileged
user. Otherwise, any control command can be sent from any unprivileged
user, but the user has to be permitted to access the ublk char device
to be controlled.

In case of UBLK_F_UNPRIVILEGED_DEV:

1) for command UBLK_CMD_ADD_DEV, it is always allowed, and user needs
to provide owner's uid/gid in this command, so that udev can set correct
ownership for the created ublk device, since the device owner uid/gid
can be queried via command of UBLK_CMD_GET_DEV_INFO.

2) for other control commands, they can only be run successfully if the
current user is allowed to access the specified ublk char device, for
running the permission check, path of the ublk char device has to be
provided by these commands.

Also add one control of command UBLK_CMD_GET_DEV_INFO2 which always
include the char dev path in payload since userspace may not have
knowledge if this device is created in unprivileged mode.

For applying this mechanism, system administrator needs to take
the following policies:

1) chmod 0666 /dev/ublk-control

2) change ownership of ublkcN & ublkbN
- chown owner_uid:owner_gid /dev/ublkcN
- chown owner_uid:owner_gid /dev/ublkbN

Both can be done via one simple udev rule.

Userspace:

	https://github.com/ming1/ubdsrv/tree/unprivileged-ublk

'ublk add -t $TYPE --un_privileged=1' is for creating one un-privileged
ublk device if the user is un-privileged.

Link: https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230106041711.914434-7-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>