lazy-pages: kill POLL_TIMEOUT

In the current model we haven't started the background page transfer until
POLL_TIMEOUT time has elapsed since the last uffd or socket event. If the
restored process will do memory access one in (POLL_TIMEOUT - eplsilon) the
filling of its memory can take ages.

This patch changes them model in the following way:
* poll for the events indefinitely until the restore is complete
* the restore completion event causes reset of the poll timeout to zero and
* starts the background transfers
* after each transfer we return to check if there are any uffd events to
handle

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>