MDEV-40667 Infinite loop on SET GLOBAL innodb_log_archive=ON

[dr-m]

log_t::set_archive(): Always invoke buf_flush_wait(wait_lsn, false) on the latest sampled get_lsn(), so that if a checkpoint is possible, one will be executed.

This fixes up #4405.

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[dr-m]

This is being tested in 75833f9 of #4817. In the Buildbot grid view the test mariabackup.huge_lsn,SERVER,strict_full_crc32 is failing most of the time on many targets. https://buildbot.mariadb.org/#/builders/1264/builds/702 is an example where this test no longer fails, even though it used to fail reliably.