lazy-pages: don't try to uffd_copy to removed memory regions

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>