mm/mempolicy: skip unnecessary synchronize_rcu()

mm/mempolicy: skip unnecessary synchronize_rcu()

By unconditionally setting wi_state to NULL and conditionally calling
synchronize_rcu(), we can save an unncessary call when there is no
old_wi_state.

Link: https://lkml.kernel.org/r/20250602162345.2595696-2-joshua.hahnjy@gmail.com
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>