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.