mount: do collect_mntinfo of external mount namespace with no for_dump
When we collect external mount namespace we don't want to dump mounts in
it, so lets remove this flag. This way we can e.g. use for_dump in
->parse() callbacks to separate in-container mounts from others.
This only affects rare case of `--ext-mount-map auto` but to be
absolutely correct let's fix it anyway.
Signed-off-by: Pavel Tikhomirov <[email protected]>