cgfsng: don't require that systemd subsystem be mounted

Note that is_crucial_subsystem still lists name=systemd.  That is
used in cgfs and cgmanager.  Cgmanager is typically setup to create
name=systemd, so it is ok.  cgfs uses is_crucial_subsystem() only
to decide whether failure to create or chown a directory should be
terminal.  That's ok, because (a) if name=systemd is not mounted then
we won't hit that, and (b) if name=systemd is mounted, then we'd
really still like to set it up for containers.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

src/lxc/cgfsng.c

@@ -407,19 +407,14 @@ static bool controller_found(struct hierarchy **hlist, char *entry)
 }
 
 /*
- * Return true if all of the controllers which we require have been
- * found.  The required list is systemd, freezer, and anything in
- * lxc.cgroup.use.
+ * Return true if all of the controllers which we require have been found.
+ * The required list is  freezer and anything in * lxc.cgroup.use.
  */
 static bool all_controllers_found(void)
 {
 	char *p, *saveptr = NULL;
 	struct hierarchy ** hlist = hierarchies;
 
-	if (!controller_found(hlist, "name=systemd")) {
-		ERROR("no systemd controller mountpoint found");
-		return false;
-	}
 	if (!controller_found(hlist, "freezer")) {
 		ERROR("no freezer controller mountpoint found");
 		return false;

src/lxc/cgroup.c

@@ -226,7 +226,12 @@ void prune_init_scope(char *cg)
 
 /*
  * Return true if this is a subsystem which we cannot do
- * without
+ * without.
+ *
+ * systemd is questionable here.  The way callers currently
+ * use this, if systemd is not mounted then it will be ignored.
+ * But if systemd is mounted, then it must be setup so that lxc
+ * can create cgroups in it, else containers will fail.
  */
 bool is_crucial_cgroup_subsystem(const char *s)
 {