It occurred to me that if vma and new_vma are one and the same, then
vma_relink_file will not do a good job of linking it after itself - in
that pretty unlikely case when move_page_tables fails.

And more generally, whenever copy_vma's vma_merge succeeds, we have no
guarantee that old vma comes before new_vma in the i_mmap lists, as we
need to satisfy Rajesh's point: that ordering is only guaranteed in the
newly allocated case.

We have to abandon the ordering method when/if we move from lists to
prio_trees, so this patch switches to the less glamorous use of
i_shared_sem exclusion, as in my prio_tree mremap.