always check whether rootfs is shared

(this expands on Dwight's recent patch, commit c597baa8f9)

After unshare(CLONE_NEWNS) and before doing any mounting, always
check whether rootfs is shared.  Otherwise template runs or clone
scripts can bleed mount activity to the host.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>