cpuacct: add a branch prediction

cpuacct_charge() is in fast-path, and checking of !cpuacct_susys.active
always returns false after cpuacct has been initialized at system boot.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>