Move failed_attempts increment into its a function (#4351)

With this change one can overwrite when the incrementation is handled without duplicating the other `valid_for_authentication` logic.

lib/devise/models/lockable.rb

@@ -101,8 +101,7 @@ module Devise
         if super && !access_locked?
           true
         else
-          self.failed_attempts ||= 0
-          self.failed_attempts += 1
+          increment_failed_attempts
           if attempts_exceeded?
             lock_access! unless access_locked?
           else
@@ -111,6 +110,11 @@ module Devise
           false
         end
       end
+      
+      def increment_failed_attempts
+        self.failed_attempts ||= 0
+        self.failed_attempts += 1
+      end
 
       def unauthenticated_message
         # If set to paranoid mode, do not show the locked message because it