x86, oprofile: BUG: using smp_processor_id() in preemptible code

Add __raw access before setting per cpu variable switch_index, to avoid
the following BUG:

[  449.166827] BUG: using smp_processor_id() in preemptible [00000000] code: modprobe/6998
[  449.166848] caller is op_nmi_init+0xf0/0x2b0 [oprofile]
[  449.166855] Pid: 6998, comm: modprobe Not tainted 2.6.27-rc5-mm1 #29
[  449.166860] Call Trace:
[  449.166872]  [<ffffffff80362d67>] debug_smp_processor_id+0xd7/0xe0
[  449.166887]  [<ffffffffa00181c0>] op_nmi_init+0xf0/0x2b0 [oprofile]
[  449.166902]  [<ffffffffa0018000>] oprofile_init+0x0/0x60 [oprofile]
[  449.166915]  [<ffffffffa00180a9>] oprofile_arch_init+0x9/0x30 [oprofile]
[  449.166928]  [<ffffffffa001801e>] oprofile_init+0x1e/0x60 [oprofile]
[  449.166937]  [<ffffffff8020903b>] _stext+0x3b/0x160
[  449.166946]  [<ffffffff80477985>] __mutex_unlock_slowpath+0xe5/0x190
[  449.166955]  [<ffffffff80262c1a>] trace_hardirqs_on_caller+0xca/0x140
[  449.166965]  [<ffffffff8026f7fc>] sys_init_module+0xdc/0x210
[  449.166972]  [<ffffffff8020b7cb>] system_call_fastpath+0x16/0x1b

Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
Acked-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>