x86, fpu: Shift "fpu_counter = 0" from copy_thread() to arch_dup_task_struct()

Cosmetic, but I think thread.fpu_counter should be initialized in
arch_dup_task_struct() too, along with other "fpu" variables. And
probably it make sense to turn it into thread.fpu->counter.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: http://lkml.kernel.org/r/20140902175730.GA21669@redhat.com
Reviewed-by: Suresh Siddha <sbsiddha@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>