MDEV-7391: rpl.rpl_semi_sync, rpl.rpl_semi_sync_after_sync_row fail in buildbot

The problem was caused by a merge error (incorrect conflict resolution) when
the MDEV-7257 patch was merged into 10.1.

The incorrect merge put two code blocks in the wrong order. This caused a race
that was seen as sporadic test failures.

(The problem was that binlog end position was updated before running
after_flush hook; this way it was possible for the binlog dump thread to send
a transaction to a slave without requesting semi-sync acknowledgement. Then
when no acknowledgement was received, semisync replication would be disabled
on the master.)

sql/log.cc

@@ -6041,14 +6041,6 @@ bool MYSQL_BIN_LOG::write(Log_event *event_info, my_bool *with_annotate)
         }
         else
         {
-          /* update binlog_end_pos so it can be read by dump thread
-           *
-           * note: must be _after_ the RUN_HOOK(after_flush) or else
-           * semi-sync-plugin might not have put the transaction into
-           * it's list before dump-thread tries to send it
-           */
-          update_binlog_end_pos(offset);
-
           /* documentation of which mutexes are (not) owned */
           mysql_mutex_assert_not_owner(&LOCK_prepare_ordered);
           mysql_mutex_assert_owner(&LOCK_log);
@@ -6065,6 +6057,14 @@ bool MYSQL_BIN_LOG::write(Log_event *event_info, my_bool *with_annotate)
           }
           else
           {
+            /* update binlog_end_pos so it can be read by dump thread
+             *
+             * note: must be _after_ the RUN_HOOK(after_flush) or else
+             * semi-sync-plugin might not have put the transaction into
+             * it's list before dump-thread tries to send it
+             */
+            update_binlog_end_pos(offset);
+
             signal_update();
             if ((error= rotate(false, &check_purge)))
               check_purge= false;