util/u_atomic: Fix the unlocked implementation.

util/u_atomic: Fix the unlocked implementation.

It was totally broken:

- p_atomic_dec_zero() was returning the negation of the expected value

- p_atomic_inc_return()/p_atomic_dec_return() was
  post-incrementing/decrementing, hence returning the old value instead
  of the new

- p_atomic_cmpxchg() was returning the new value on success, instead of
  the old

It is clear this never used in the past. I wonder if it wouldn't be better to
yank it altogether.

Reviewed-by: Matt Turner <mattst88@gmail.com>