nommu: pass NULL argument to vma_iter_prealloc()

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