x86/oprofile: disable preemption in nmi_shutdown

fix:

    BUG: using smp_processor_id() in preemptible [00000000] code: oprofiled/27301
    caller is nmi_shutdown+0x11/0x60
    Pid: 27301, comm: oprofiled Not tainted 2.6.26-rc7 #25
     [<c028a90d>] debug_smp_processor_id+0xbd/0xc0
     [<c045fba1>] nmi_shutdown+0x11/0x60
     [<c045dd4a>] oprofile_shutdown+0x2a/0x60

Note that we don't need this for the other functions, since they are all
called with on_each_cpu() (which disables preemption for us anyway).

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Cc: Philippe Elie <phil.el@wanadoo.fr>
Cc: oprofile-list@lists.sf.net
Cc: Johannes Weiner <hannes@saeurebad.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>