Try to add a custom prefixe on your layout like:
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto" >
<item android:id="@+id/action_search"
android:title="@string/search"
android:icon="@drawable/ic_action_search"
app:showAsAction="ifRoom|collapseActionView"
app:actionViewClass="android.support.v7.widget.SearchView" />
</menu>
See the documentation, in the example, you must to add a custom prefixe (as "app" in the code above).
Also, instead of:
MenuItem searchItem = menu.findItem(R.id.action_search);
mSearchView = (SearchView) searchItem.getActionView();
Try this:
MenuItem searchItem = menu.findItem(R.id.action_search);
// with MenuItemCompat instead of your MenuItem
mSearchView = (SearchView) MenuItemCompat.getActionView(searchItem);
Let me know if this helps.