Remove WARN_ON in smp_call_function. it breaks the machine check
handling and there is no good way to enable interrupts here.

 Call Trace:<ffffffff8011cb33>{smp_call_function+115} <ffffffff8011cbf9>{smp_send_stop+25}
 <ffffffff80138012>{panic+274} <ffffffff801187e0>{do_machine_check+0}
 <ffffffff80118c3a>{do_machine_check+1114} <ffffffff80111663>{machine_check+127}
 <ffffffff803434d7>{net_rx_action+215}  <EOE> <IRQ> <ffffffff8013c8e3>{__do_softirq+83}
   <ffffffff8013c975>{do_softirq+53} <ffffffff80113cdd>{do_IRQ+317}
 <<ffffffff8010f3b0>{default_idle+0} <ffffffff80110b81>{ret_from_intr+0}
     <EOI> <ffffffff8010f3d0>{default_idle+32} <ffffffff8010f86a>{cpu_idle+26}
  <ffffffff8058f886>{start_kernel+502} <ffffffff8058f1e0>{_sinittext+48
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>