[S390] cputime: fix lowcore initialization on cpu hotplug

On (initial) cpu hotplug the lowcore values for user_timer and
system_timer don't get initialized like they would get on each
process schedule.
On initial start of secondary cpus this leads to the situation
where per thread user/system_timer values are larger than the
corresponding contents of the lowcore. When later calculating
time spent in user/system context the result can be negative.

So for cpu hotplug we should manually initialize lowcore values.

Fixes this bug:

Kernel BUG at 000ec080 [verbose debug info unavailable]
fixpoint divide exception: 0009 [#1] PREEMPT SMP
Modules linked in:
CPU: 10 Not tainted 2.6.28 #4
Process sysctl (pid: 975, task: 3fa752e0, ksp: 3fbebca0)
Krnl PSW : 070c1000 800ec080 (show_stat+0x390/0x5fc)
           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:1 PM:0
Krnl GPRS: 7fffffff fefc7ce5 3faec080 003879ae
           00000001 01388000 7fffffff 01388000
           00000000 00000000 0049ad50 3fbebcf8
           01388000 002f51a8 800ec1fe 3fbebcf8
Krnl Code: 800ec076: 9001b188           stm     %r0,%r1,392(%r11)
           800ec07a: 9801b0c0           lm      %r0,%r1,192(%r11)
           800ec07e: 1d05               dr      %r0,%r5
          >800ec080: 9001b0c0           stm     %r0,%r1,192(%r11)
           800ec084: 5860b0c4           l       %r6,196(%r11)
           800ec088: 1806               lr      %r0,%r6
           800ec08a: 8c800001           srdl    %r8,1
           800ec08e: 1d87               dr      %r8,%r7
Call Trace:
([<00000000000ec1ee>] show_stat+0x4fe/0x5fc)
 [<00000000000c13e8>] seq_read+0xc4/0x3ac
 [<00000000000e4796>] proc_reg_read+0x6e/0x9c
 [<00000000000a6a44>] vfs_read+0x78/0x100
 [<00000000000a6ba8>] sys_read+0x40/0x80
 [<00000000000234a8>] sysc_do_restart+0x1a/0x1e

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>