Currently we use pagemap to check if we should copy a page into process
address space or zero it. The lazy_iov'ecs can be used instead. If a
lazy_iov covers the faulting address, we should go ahead and read the page
and copy it. If there is not lazy_iov for the faulting address, just zero
it immediately.
Searching lazy_iov's rather than pagemap will also simplify upcoming
handling of UFFD_EVENT_REMAP.

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>