It is possible that when pages request from the remove source arrive, part
of the memory range covered by the request would be already gone because of
madvise(MADV_DONTNEED), mremap() etc.
Ensure we are not trying to uffd_copy more than we are allowed.
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>