mnt: Shorten the mntns dumping loop

We currently have all mouninfo-s from all mnt namespaces collected
in one big list. On dump we scan through it to find the namespaces
we need to dump.

This can be optimized by walking the list of namespaces instead.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>