ipc: Fix huge ipcshm segments data dump/restore

The ipcns-shm images are buffered and contain raw dumps of memory
contents. If the segment is huge, the bfd engine overflows and
doesn't write data into image. This is not nice by itself, but
huge images shouldn't use bfd to avoid double buffering.

Also, in the future, this image should be fixed not to keep mem
dumps in one image with object.

https://github.com/xemul/criu/issues/287



Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>