From: Hugh Dickins <hugh@veritas.com>

Partial rewrite of mremap's move_vma.  Rajesh Venkatasubramanian has pointed
out that vmtruncate could miss ptes, leaving orphaned pages, because move_vma
only made the new vma visible after filling it.  We see no good reason for
that, and time to make move_vma more robust.

Removed all its vma merging decisions, leave them to mmap.c's vma_merge, with
copy_vma added.  Removed duplicated is_mergeable_vma test from vma_merge, and
duplicated validate_mm from insert_vm_struct.

move_vma move from old to new then unmap old; but on error move back from new
to old and unmap new.  Don't unwind within move_page_tables, let move_vma
call it explicitly to unwind, with the right source vma.  Get the
VM_ACCOUNTing right even when the final do_munmap fails.