page-pipe: allow to share pipes between page pipe buffers

Now criu create a new pipe buffer, if a previous one has another set of
flags. In this case, a pipe is not full and we can use it for the
next page buffer.

We need 88 pipes to pre-dump the zdtm/static/fork test without this
patch, and we need only 17 pipes with this patch.

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