cpuset: avoid unnecessary sched domains rebuilding

When changing 'sched_relax_domain_level', don't rebuild sched domains if
'cpus' is empty or 'sched_load_balance' is not set.

Also make the comments of rebuild_sched_domains() more readable.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>