mount: separate collect_mnt_ns from dump_mnt_ns

We are going to support nested mntns, so the global mntinfo_tree
variable are useless and information about tree should be connected
to a proper namespace.

But when we don't dump mntns, we need to collect mounts for the current
mntns.

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