nommu: pass NULL argument to vma_iter_prealloc()

When deleting a vma entry from a maple tree, it has to pass NULL to
vma_iter_prealloc() in order to calculate internal state of the tree, but
it passed a wrong argument.  As a result, nommu kernels crashed upon
accessing a vma iterator, such as acct_collect() reading the size of vma
entries after do_munmap().

This commit fixes this issue by passing a right argument to the
preallocation call.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: b5df092 ("mm: set up vma iterator for vma_iter_prealloc() calls")
Signed-off-by: Hajime Tazaki <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/nommu.c

@@ -573,7 +573,7 @@ static int delete_vma_from_mm(struct vm_area_struct *vma)
 	VMA_ITERATOR(vmi, vma->vm_mm, vma->vm_start);
 
 	vma_iter_config(&vmi, vma->vm_start, vma->vm_end);
-	if (vma_iter_prealloc(&vmi, vma)) {
+	if (vma_iter_prealloc(&vmi, NULL)) {
 		pr_warn("Allocation of vma tree for process %d failed\n",
 		       current->pid);
 		return -ENOMEM;