util/u_atomic: fix return type of p_atomic_{inc,dec}_return() and p_atomic_{cmp,}xchg()

We're trying to cast the return type to the type of the var, but instead
we were casting `sizeof(*v)`.

Fixes: 6df72e97 ("util: Make u_atomic.h typeless.")
Fixes: 0a7f17cf ("util/u_atomic: add p_atomic_xchg")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
(cherry picked from commit aaab7003)