ublk: use READ_ONCE() to read struct ublksrv_ctrl_cmd

In the Linux kernel, the following vulnerability has been resolved: ublk: use READ_ONCE() to read struct ublksrv_ctrl_cmd struct ublksrv_ctrl_cmd is part of the io_uring_sqe, which may lie in userspace-mapped memory. It's racy to access its fields with normal loads, as userspace may write to them concurrently. Use READ_ONCE() to copy the ublksrv_ctrl_cmd from the io_uring_sqe to the stack. Use the local copy in place of the one in the io_uring_sqe.