The UNMAP event is generated by userfaultfd when a VMA (or a part of it) is
unmapped from the process address space. We won't receive #PF's at the
unmapped range, but we need to make sure we are not trying to fill that
range at handle_remaining_pages.
Note, that the VMA is gone, so there is no sense to unregister uffd from
it.

travis-ci: success for lazy-pages: add non-#PF events handling (rev2)
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>