CVE-2026-9771 Missing device-pointer validation in flash_copy() syscall allows userspace privilege escalation

The flashcopy system call is verified by zvrfyflashcopy in drivers/flash/flashutil.c. On builds with CONFIGUSERSPACE enabled, this handler is the kernel-side trust boundary for a user-mode caller. Prior to the fix it validated only the output buffer KSYSCALLMEMORYWRITE and passed the two struct...