sched: fix double kfree in failure path

It's not the responsibility of init_rootdomain() to free root_domain
allocated by alloc_rootdomain().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>