Take SAI API lock when registering switch event callback

Summary:
While trying to make sure no regression in Sai Replayer refactoring, noticed that a few SAI API calls are interleaving with each other. E.g.

```
memset(s_a,0,ATTR_SIZE*4096);
s_a[0].id=536870926;
rv=switch_api->set_switch_attribute(9840384019236126720U,s_a);
memset(s_a,0,ATTR_SIZE*4096);
s_a[0].id=120;
rv=switch_api->set_switch_attribute(9840384019236126720U,s_a);
rvCheck(rv,0,1952);

rvCheck(rv,0,1953);
```

This is because when we register callbacks, the call directly goes to SDK instead of going through Sai API.

To fix this, also take the SaiAPILock while registering callback.

Reviewed By: daiwei1983

Differential Revision:
D85835200

Privacy Context Container: L1297311

fbshipit-source-id: 91c5c23810be7988d2bc9cfb73df1a68c6bf2e29

Author: Ron He

fboss/agent/hw/sai/api/bcm/SwitchApi.cpp

@@ -341,25 +341,38 @@ void SwitchApi::registerSwitchEventCallback(
 #endif
     eventAttr.value.u32list.count = events.size();
     eventAttr.value.u32list.list = events.data();
-    auto rv = _setAttribute(id, &eventAttr);
-    saiLogError(rv, ApiType, "Unable to register parity error switch events");
 
-    // Register switch event callback function
-    rv = _setAttribute(id, &attr);
-    saiLogError(
-        rv, ApiType, "Unable to register parity error switch event callback");
+    {
+      auto g{SaiApiLock::getInstance()->lock()};
+      auto rv = _setAttribute(id, &eventAttr);
+      saiLogError(rv, ApiType, "Unable to register parity error switch events");
+    }
+
+    {
+      // Register switch event callback function
+      auto g{SaiApiLock::getInstance()->lock()};
+      auto rv = _setAttribute(id, &attr);
+      saiLogError(
+          rv, ApiType, "Unable to register parity error switch event callback");
+    }
   } else {
     // This is reverse of the registration sequence.
     // First, unregister callback, then unregister events.
 
-    // First unregister callback function
-    auto rv = _setAttribute(id, &attr);
-    saiLogError(rv, ApiType, "Unable to unregister TAM event callback");
-
-    // Then unregister switch events
-    eventAttr.value.u32list.count = 0;
-    rv = _setAttribute(id, &eventAttr);
-    saiLogError(rv, ApiType, "Unable to unregister switch events");
+    {
+      auto g{SaiApiLock::getInstance()->lock()};
+      // First unregister callback function
+      auto rv = _setAttribute(id, &attr);
+      saiLogError(rv, ApiType, "Unable to unregister TAM event callback");
+    }
+
+    {
+      auto g{SaiApiLock::getInstance()->lock()};
+      // Then unregister switch events
+      eventAttr.value.u32list.count = 0;
+      auto rv = _setAttribute(id, &eventAttr);
+      saiLogError(rv, ApiType, "Unable to unregister switch events");
+    }
   }
 #endif
 }