This check was added by commit aebfabb5 "mnt: add --ext-mount-map
auto option", but unless I am totally confused it actually belongs
to the (already reverted) 246367e4 "add walk_all flag to
walk_namespaces".

Remove it. It is no longer needed and it was very unobvious.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>