ublk: add UBLK_CMD_TRY_STOP_DEV command

ublk: add UBLK_CMD_TRY_STOP_DEV command

Add a best-effort stop command, UBLK_CMD_TRY_STOP_DEV, which only stops a
ublk device when it has no active openers.

Unlike UBLK_CMD_STOP_DEV, this command does not disrupt existing users.
New opens are blocked only after disk_openers has reached zero; if the
device is busy, the command returns -EBUSY and leaves it running.

The ub->block_open flag is used only to close a race with an in-progress
open and does not otherwise change open behavior.

Advertise support via the UBLK_F_SAFE_STOP_DEV feature flag.

Signed-off-by: Yoav Cohen <yoav@nvidia.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>