logout redirect only from admin page

For your if condition, use the is_admin function to determine if the user is currently viewing a backend administrative dashboard page or are on the frontend of the site.

So your code above would resemble:

add_action('wp_logout','redirect_after_logout');

function redirect_after_logout(){
  if( is_admin() ) {
    wp_redirect( home_url() );
    exit();
  }
}

For more information, see the codex documentation at https://codex.wordpress.org/Function_Reference/is_admin