drm/ttm: Fix accesses through vmas with only partial coverage

commit d3867355 upstream.

VMAs covering a bo but that didn't start at the same address space offset as
the bo they were mapping were incorrectly generating SEGFAULT errors in
the fault handler.
Reported-by: Joseph Dolinak <kanilo2@yahoo.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
[bwh: Backported to 3.2: drm_vma_node_start() is open-coded;
 vma_pages() was open-coded]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/ttm/ttm_bo_vm.c

@@ -144,9 +144,9 @@ static int ttm_bo_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	}
 
 	page_offset = ((address - vma->vm_start) >> PAGE_SHIFT) +
-	    bo->vm_node->start - vma->vm_pgoff;
-	page_last = ((vma->vm_end - vma->vm_start) >> PAGE_SHIFT) +
-	    bo->vm_node->start - vma->vm_pgoff;
+		vma->vm_pgoff - bo->vm_node->start;
+	page_last = vma_pages(vma) + vma->vm_pgoff -
+		bo->vm_node->start;
 
 	if (unlikely(page_offset >= bo->num_pages)) {
 		retval = VM_FAULT_SIGBUS;