MDEV-39061 mariadb-backup compatible wrapper for BACKUP SERVER

backup_innodb.cc: record the backup end LSN as the current log LSN
(log_sys.get_lsn()) instead of the flushed LSN, so a committed
transaction whose redo has not yet been flushed to disk is still
covered by the backup's recovery target.

Author: Thirunarayanan

storage/innobase/handler/backup_innodb.cc

@@ -231,7 +231,7 @@ class InnoDB_backup
         goto skip_log_dup;
       backup_context &ctx= trx->lock.backup;
       ut_ad(ctx.max_first_lsn);
-      ctx.last_lsn= log_sys.get_flushed_lsn(std::memory_order_relaxed);
+      ctx.last_lsn= log_sys.get_lsn();
       while (!logs.empty())
       {
         lsn_t lsn{logs.back()};