fbdev: udlfb: add vm_ops to dlfb_ops_mmap to prevent use-after-free

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

fbdev: udlfb: add vm_ops to dlfb_ops_mmap to prevent use-after-free

dlfb_ops_mmap() uses remap_pfn_range() to map vmalloc framebuffer pages

to userspace but sets no vm_ops on the VMA. This means the kernel cannot

track active mmaps. When dlfb_realloc_framebuffer() replaces the backing

buffer via FBIOPUT_VSCREENINFO, existing mmap PTEs are not invalidated.

On USB disconnect, dlfb_ops_destroy() calls vfree() on the old pages

while userspace PTEs still reference them, resulting in a use-after-free:

the process retains read/write access to freed kernel pages.

Add vm_operations_struct with open/close callbacks that maintain an

atomic mmap_count on struct dlfb_data. In dlfb_realloc_framebuffer(),

check mmap_count and return -EBUSY if the buffer is currently mapped,

preventing buffer replacement while userspace holds stale PTEs.

Tested with PoC using dummy_hcd + raw_gadget USB device emulation.