accel/tcg/plugin: Tidy plugin_gen_disable_mem_helpers

Here we are creating a temp whose value needs to be replaced,
but always storing NULL into CPUState.plugin_mem_cbs.
Use tcg_constant_ptr(0) explicitly.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/accel/tcg/plugin-gen.c b/accel/tcg/plugin-gen.c
index 9b793ac..c42a436 100644
--- a/accel/tcg/plugin-gen.c
+++ b/accel/tcg/plugin-gen.c
@@ -630,8 +630,6 @@
 /* called before finishing a TB with exit_tb, goto_tb or goto_ptr */
 void plugin_gen_disable_mem_helpers(void)
 {
-    TCGv_ptr ptr;
-
     /*
      * We could emit the clearing unconditionally and be done. However, this can
      * be wasteful if for instance plugins don't track memory accesses, or if
@@ -644,10 +642,8 @@
     if (!tcg_ctx->plugin_tb->mem_helper) {
         return;
     }
-    ptr = tcg_const_ptr(NULL);
-    tcg_gen_st_ptr(ptr, cpu_env, offsetof(CPUState, plugin_mem_cbs) -
-                                 offsetof(ArchCPU, env));
-    tcg_temp_free_ptr(ptr);
+    tcg_gen_st_ptr(tcg_constant_ptr(NULL), cpu_env,
+                   offsetof(CPUState, plugin_mem_cbs) - offsetof(ArchCPU, env));
 }
 
 static void plugin_gen_tb_udata(const struct qemu_plugin_tb *ptb,