[MIPS] Fix futex_atomic_op_inuser.

I found that NPTL's pthread_cond_signal() does not work properly on
kernels compiled by gcc 4.1.x.  I suppose inline asm for
__futex_atomic_op() was wrong.  I suppose:

1. "=&r" constraint should be used for oldval.
2. Instead of "r" (uaddr), "=R" (*uaddr) for output and "R" (*uaddr)
   for input should be used.
3. "memory" should be added to the clobber list.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>