FIX: calculate page if page param is not given to TopicView (#10953)

Currently, when page param is not given to TopicView we calculate page for canonical_path, however, it is skipped for next_path.

We should use the same calculation to define page, so next page URL will be accurate. Currently if you [view source of meta post](view-source:https://meta.discourse.org/t/post-rate-limit-trigger-for-a-topic-thats-heating-up/98294/46) you will see:

```
<link rel="canonical" href="https://meta.discourse.org/t/post-rate-limit-trigger-for-a-topic-thats-heating-up/98294?page=3" />
<link rel="next" href="/t/post-rate-limit-trigger-for-a-topic-thats-heating-up/98294?page=2">
```

lib/topic_view.rb

@@ -66,7 +66,6 @@ class TopicView
     end
 
     @post_number = [@post_number.to_i, 1].max
-    @page = [@page.to_i, 1].max
 
     @include_suggested = options.fetch(:include_suggested) { true }
     @include_related = options.fetch(:include_related) { true }
@@ -79,6 +78,8 @@ class TopicView
 
     @limit ||= @chunk_size
 
+    @page = @page.to_i > 1 ? @page.to_i : calculate_page
+
     setup_filtered_posts
 
     @initial_load = true
@@ -118,15 +119,7 @@ class TopicView
       return topic_embed.embed_url if topic_embed
     end
     path = relative_url.dup
-    path <<
-      if @page > 1
-        "?page=#{@page}"
-      else
-        posts_count = is_mega_topic? ? @post_number : unfiltered_posts.where("post_number <= ?", @post_number).count
-        page = ((posts_count - 1) / @limit) + 1
-        page > 1 ? "?page=#{page}" : ""
-      end
-
+    path << ((@page > 1) ? "?page=#{@page}" : "")
     path
   end
 
@@ -640,6 +633,11 @@ class TopicView
 
   private
 
+  def calculate_page
+    posts_count = is_mega_topic? ? @post_number : unfiltered_posts.where("post_number <= ?", @post_number).count
+    ((posts_count - 1) / @limit) + 1
+  end
+
   def get_sort_order(post_number)
     sql = <<~SQL
       SELECT posts.sort_order