sched: accounting regression since rc1

Fix the accounting regression for CONFIG_VIRT_CPU_ACCOUNTING.  It
reverts parts of commit b27f03d4 by
converting fs/proc/array.c back to cputime_t.  The new functions
task_utime and task_stime now return cputime_t instead of clock_t.  If
CONFIG_VIRT_CPU_ACCOUTING is set, task->utime and task->stime are
returned directly instead of using sum_exec_runtime.

Patch is tested on s390x with and without VIRT_CPU_ACCOUTING as well as
on i386.

[ mingo@elte.hu: cleanups, comments. ]

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>