From: Hugh Dickins <hugh@veritas.com>

Subtle point from Rajesh Venkatasubramanian: when mremap's move_vma fails and
so rewinds, before moving the file-based ptes back, we must move new_vma
before old vma in the i_mmap or i_mmap_shared list, so that when racing
against vmtruncate we cannot propagate pages to be truncated back from
new_vma into the just cleaned old_vma.