Do not show usable memory if KDUMP_AUTO_RESIZE is set

The number is confusing, because it only shows how much memory is
available at early boot (before kdump-early.service is started). It
is impossible to know exactly how much memory will be allocated at
that time. Rather than making a wild guess, I prefer to avoid
showing any value, so people at least realize that something works
very differently when this option is selected.

src/include/kdump/uifunctions.rb

@@ -1333,7 +1333,11 @@ def allocated_memory
 
     # Updates the free memory displayed in the UI
     def update_usable_memory
-      value = Kdump.total_memory - allocated_memory
+      value = if UI.QueryWidget(Id(:auto_resize), :Value)
+        "---"
+      else
+        Kdump.total_memory - allocated_memory
+      end
       UI.ChangeWidget(Id("usable_memory"), :Value, value.to_s)
     end
 
@@ -1382,6 +1386,8 @@ def HandleKdumpMemory(_key, event)
           UI.ChangeWidget(Id(ret), :Value, send(ret.to_sym) + remaining)
         end
         update_usable_memory
+      elsif ret == :auto_resize
+        update_usable_memory
       end
 
       nil