fix cgpath test

Commit 1ea59ad28 sets memory.use_hierarchy, which means that this test
cannot use memory.swappiness as its dummy cgroup item to set/unset since
writing to it with use_hierarchy set gets -EINVAL. Change test to use
memory.soft_limit_in_bytes instead.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
coverity 1097616: don't deref NULL when subsystem doesn't have a '.'

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