pipe: prevent dumping pipes in the packetized mode

This mode was introduced less than year ago:

commit 9883035ae7edef3ec62ad215611cb8e17d6a1a5d
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun Apr 29 13:12:42 2012 -0700

    pipes: add a "packetized pipe" mode for writing

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>