udmabuf: add back sanity check

Check vm_fault->pgoff before using it.  When we removed the warning, we
also removed the check.

Fixes: 7b26e4e2 ("udmabuf: drop WARN_ON() check.")
Reported-by: zdi-disclosures@trendmicro.com
Suggested-by: NLinus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/dma-buf/udmabuf.c

@@ -32,8 +32,11 @@ static vm_fault_t udmabuf_vm_fault(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	struct udmabuf *ubuf = vma->vm_private_data;
+	pgoff_t pgoff = vmf->pgoff;
 
-	vmf->page = ubuf->pages[vmf->pgoff];
+	if (pgoff >= ubuf->pagecount)
+		return VM_FAULT_SIGBUS;
+	vmf->page = ubuf->pages[pgoff];
 	get_page(vmf->page);
 	return 0;
 }