lazy-pages: replace crappy num_children with simple reference counting

The purpose of the num_children field in 'struct lazy_pages_info' was to
prevent closing the page-read while there are still active processes that
share it. It did work for the case when handling of the child processes
finished before the parent process. However, if the parent lpi is closed
first, we've got a dangling pointer at lpi->parent.
The obvious solution is to use simple reference counting.

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