From: Hugh Dickins <hugh@veritas.com>

mremap's move_vma VM_LOCKED case was still wrong.

If the do_munmap unmaps a part of new_vma, then its vm_start and vm_end
from before cannot both be the right addresses for the make_pages_present
range, and may BUG() there.

We need [new_addr, new_addr+new_len) to be locked down; but
move_page_tables already transferred the locked pages [new_addr,
new_addr+old_len), and they're either held in a VM_LOCKED vma throughout,
or temporarily in no vma: in neither case can be swapped out, so no need to
run over that range again.