fbdev: fix potential buffer overflow in do_register_framebuffer()

In the Linux kernel, the following vulnerability has been resolved: fbdev: fix potential buffer overflow in do_register_framebuffer() The current implementation may lead to buffer overflow when: 1. Unregistration creates NULL gaps in registered_fb[] 2. All array slots become occupied despite num_registered_fb < FB_MAX 3. The registration loop exceeds array bounds Add boundary check to prevent registered_fb[FB_MAX] access.