page-pipe: Don't increase the page_pipe::nr_pipes if we reuse pipes

The page-pipe buffers may be reused once queued pages are
dumped, but we happen to increase page_pipe::nr_pipes
all the timer, regardless the fact where page buffer
came from.

In worst scenario this may lead to incorrect -EAGAIN returned
from page_pipe_grow forcing calling code to create new
pipes. This is not critical but should be fixed.

In other words page_pipe::nr_pipes must track _really_
created pipes.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>