attach: handle apparmor transitions in !NEWNS cases

If we're not attaching to the mount ns , then don't enter the
container's apparmor policy.  Since we're running binaries from the host
and not the container, that actually seems the sane thing to do (besides
also the lazier thing).

If we dont' do this patch, then we will need to move the apparmor attach
past the procfs remount, will need to also mount securityfs if available,
and for the !remount_proc_sys case we'll want to mount those just long
enough to do the apparmor transition.

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