page-pipe: Update format for PAGE_SIZE

 | page-pipe.c:230:4: error: format ‘%zu’ expects argument of type ‘size_t’, but argument 4 has type ‘long unsigned int’ [-Werror=format]
 | page-pipe.c:237:3: error: format ‘%zu’ expects argument of type ‘size_t’, but argument 4 has type ‘long unsigned int’ [-Werror=format]

This is because PAGE_SIZE is defined as unsigned long (same
as kernel does).

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>