lazy-pages: drop_iovs: mark iov as not queued

If we receive only part of the IOV from the page-server we recalculate the
IOV so it will point to the area we still have to fetch. During the split,
the IOV covering the remaining area may remain marked as 'queued' and we'll
never retry fetching it.
Marking the IOV as not queued will ensure its pages will be requested
again.

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