CVE-2025-38478: comedi: Fix initialization of data for instructions that write to subdevice

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

comedi: Fix initialization of data for instructions that write to subdevice

Some Comedi subdevice instruction handlers are known to access instruction data elements beyond the first insn->n elements in some cases. The doinsnioctl() and doinsnlistioctl() functions allocate at least MINSAMPLES (16) data elements to deal with this, but they do not initialize all of that. For Comedi instruction codes that write to the subdevice, the first insn->n data elements are copied from user-space, but the remaining elements are left uninitialized. That could be a problem if the subdevice instruction handler reads the uninitialized data. Ensure that the first MINSAMPLES elements are initialized before calling these instruction handlers, filling the uncopied elements with 0. For doinsnlistioctl(), the same data buffer elements are used for handling a list of instructions, so ensure the first MINSAMPLES elements are initialized for each instruction that writes to the subdevice.