page-pipe: split dumping memory on chunks (v3)

The problem is that vmsplice() to a big pipe fails very often.

The kernel allocates a linear chunk of memory for pipe buffer
descriptos, but a big allocation in kernel can fail.

So we need to restrict maximal capacity of pipes. But the number of
pipes is restricted too, so we need to split dumping memory on chunks.

In this patch we calculates the pipe size for which vmsplice() will not
fail.

v2: s/batch/chunk and a few other small fixes
v3: Remove callbacks from page_pipes and reuse pipes
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>