don't leak the rootfs.pin fd into the container

Only the container parent needs to keep that fd open.  Close it
as soon as the container's first task is spawned.  Else it can
show up in /proc/$$/fd in the container.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>