Remove redundant update of txn_state_ in transaction Prepare (#6778)

Summary:
When  expiration is set in a pessimistic transaction, `txn_state_` is already updated to `AWAITING_PREPARE` in the `if (expiration_time_ > 0)` block, there is  no need to update the state in `if (can_prepare)` block again.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6778

Test Plan: make check

Reviewed By: lth

Differential Revision: D21335319

Pulled By: cheng-chang

fbshipit-source-id: 251d634cc7d1a0e86e673a59f0bda8584da5a35f

utilities/transactions/pessimistic_transaction.cc

@@ -191,11 +191,11 @@ Status PessimisticTransaction::Prepare() {
                                                       AWAITING_PREPARE);
   } else if (txn_state_ == STARTED) {
     // expiration and lock stealing is not possible
+    txn_state_.store(AWAITING_PREPARE);
     can_prepare = true;
   }
 
   if (can_prepare) {
-    txn_state_.store(AWAITING_PREPARE);
     // transaction can't expire after preparation
     expiration_time_ = 0;
     assert(log_number_ == 0 ||