page-xfer: Handle partial splicing

In case if pipe buffers are full the splicing
may wakeup reader first with incomplete data
transfer. Thus splice data in cycle.

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