mm/mremap: write-lock VMA while remapping it to a new address range

Write-lock VMA as locked before copying it and when copy_vma produces a
new VMA.

Link: https://lkml.kernel.org/r/20230227173632.3292573-18-surenb@google.com


Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Laurent Dufour <laurent.dufour@fr.ibm.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>