Remove a memory leak on error path.

Only try to initialize cpuset if cgroup.clonechildren does not exist.

Bump the max value we read from cpuset.{cpus,mems} to 1024.

If cpuset.cpus or .mems is already initialized but is too long, don't fail.

If parent's cpuset.cpus or .mems is too long, record an error and fail.
If anyone actually runs into this, we can simply allocate the required
length as needed, but we don't expect anyone to run into this.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>