parasite: Always switch netns for transport socket

If we're doing the pre-dump command we don't have the tasks'
IDs collected, so we have the current_ns_mask uninitilized.
Thus we create parasite transport socket in criu's namespace
always and pre-dump of task, sitting in foreing net namespace
may hang.

From the "performance" point of view, it's easier to always
create transport socket in victim's namespace, rahter than
getting the current_ns_mask.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>