ublk: fail to start device if queue setup is interrupted

In the Linux kernel, the following vulnerability has been resolved:

ublk: fail to start device if queue setup is interrupted

In ublk_ctrl_start_dev(), if wait_for_completion_interruptible() is interrupted by signal, queues aren't setup successfully yet, so we have to fail UBLK_CMD_START_DEV, otherwise kernel oops can be triggered.

Reported by German when working on qemu-storage-deamon which requires single thread ublk daemon.