Merge branch 'lfs-token-race-condition-fix' into 'master'

Fix race condition on LFS Token

## What does this MR do?

 Fixes a race condition that can be triggered if the token expires right after we retrieve it, but before we can set the new expiry time. 

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6551#note_16190569

Thanks for catching this so quickly @jacobvosmaer-gitlab 

cc @DouweM

See merge request !6592

lib/gitlab/lfs_token.rb

@@ -20,13 +20,8 @@ module Gitlab
     def token
       Gitlab::Redis.with do |redis|
         token = redis.get(redis_key)
-
-        if token
-          redis.expire(redis_key, EXPIRY_TIME)
-        else
-          token = Devise.friendly_token(TOKEN_LENGTH)
+        token ||= Devise.friendly_token(TOKEN_LENGTH)
         redis.set(redis_key, token, ex: EXPIRY_TIME)
-        end
 
         token
       end