bpf: Add bpf_selem_free_rcu callback

Add bpf_selem_free_rcu() callback to do the kfree() instead
of using kfree_rcu. It is a preparation work for using
bpf_mem_cache_alloc/free in a later patch.
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20230308065936.1550103-9-martin.lau@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/bpf_local_storage.c

@@ -109,15 +109,20 @@ static void bpf_local_storage_free_rcu(struct rcu_head *rcu)
 		kfree_rcu(local_storage, rcu);
 }
 
-static void bpf_selem_free_trace_rcu(struct rcu_head *rcu)
+static void bpf_selem_free_rcu(struct rcu_head *rcu)
 {
 	struct bpf_local_storage_elem *selem;
 
 	selem = container_of(rcu, struct bpf_local_storage_elem, rcu);
+	kfree(selem);
+}
+
+static void bpf_selem_free_trace_rcu(struct rcu_head *rcu)
+{
 	if (rcu_trace_implies_rcu_gp())
-		kfree(selem);
+		bpf_selem_free_rcu(rcu);
 	else
-		kfree_rcu(selem, rcu);
+		call_rcu(rcu, bpf_selem_free_rcu);
 }
 
 /* local_storage->lock must be held and selem->local_storage == local_storage.
@@ -174,7 +179,7 @@ static bool bpf_selem_unlink_storage_nolock(struct bpf_local_storage *local_stor
 	if (!reuse_now)
 		call_rcu_tasks_trace(&selem->rcu, bpf_selem_free_trace_rcu);
 	else
-		kfree_rcu(selem, rcu);
+		call_rcu(&selem->rcu, bpf_selem_free_rcu);
 
 	if (rcu_access_pointer(local_storage->smap) == smap)
 		RCU_INIT_POINTER(local_storage->smap, NULL);