predump: Collect mnt and net namespaces properly

On pre-dump we collect only two namespaces -- the mnt one
for criu and mnt one again for root task.

This is not correct. We need all mount namespaces to make
the irmap generation work properly and we need all net
namespaces to have parasite sockets created.

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