A race condition exists within smp_call_function_many() when called from
smp_flush_tlb_mm().  On rare occasions the cpu_vm_mask can be cleared
while smp_call_function_many is executing, occasionally resulting in a
hung process.

Make a copy of the mask prior to calling smp_call_function_many().
Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>