FIX: properly count DistributedMutex locking attempts

When originally introduced, `attempts` was only used in the read-only check
context.

With the introduction of the exponential backoff in cda370db, `attempts` was
also used to count loop iterations, but was left inside the if block instead of
being incremented every loop, meaning the exponential backoff was only
happening when the site was recently readonly.

Co-authored-by: jbrw <jamie.wilson@discourse.org>

lib/distributed_mutex.rb

@@ -88,14 +88,11 @@ class DistributedMutex
       # Exponential backoff, max duration 1s
       interval = attempts < 10 ? (0.001 * 2**attempts) : 1
       sleep interval
+      attempts += 1
 
       # in readonly we will never be able to get a lock
-      if @using_global_redis && Discourse.recently_readonly?
-        attempts += 1
-
-        if attempts > CHECK_READONLY_ATTEMPTS
-          raise Discourse::ReadOnly
-        end
+      if @using_global_redis && Discourse.recently_readonly? && attempts > CHECK_READONLY_ATTEMPTS
+        raise Discourse::ReadOnly
       end
     end
   end