core: let selinux_setup() load policy more than once

core: let selinux_setup() load policy more than once

When you switch-root into a new root that has SELinux policy, you're
supposed to to run selinux_init_load_policy() to set up SELinux and load
policy. Normally this gets handled by selinux_setup().

But if SELinux was already initialized, selinux_setup() skips loading
policy and returns 0. So if you load policy normally, and then you
switch-root to a new root that has new policy, selinux_setup() never
loads the new policy. What gives?

As far as I can tell, this check is an artifact of how selinux_setup()
worked when it was first written (see commit c4dcdb9 / systemd v12):

  * when systemd starts, run selinux_setup()
  * if selinux_setup() loads policy OK, restart systemd

So the "if policy already loaded, skip load and return 0" check was
there to prevent an infinite re-exec loop.

Modern systemd only calls selinux_setup() on initial load and after
switch-root, and selinux_setup() no longer restarts systemd, so we don't
need that check to guard against the infinite loop anymore.

So: this patch removes the "return 0", thus allowing selinux_setup() to
actually perform SELinux setup after switch-root.

We still want to check to see if SELinux is initialized, because if
selinux_init_load_policy() fails *but* SELinux is initialized that means
we still have (old) policy active. So we don't need to halt if
enforce=1.