MDEV-23855 broke the handling of innodb_flush_sync=OFF.
That parameter is supposed to limit the page write rate
in case the log capacity is being exceeded and log checkpoints
are needed.

With this fix, the following should pass:
./mtr --mysqld=--loose-innodb-flush-sync=0

One of our best regression tests for page flushing is
encryption.innochecksum. With innodb_page_size=16k and
innodb_flush_sync=OFF it would likely hang without this fix.

log_sys.last_checkpoint_lsn: Declare as Atomic_relaxed<lsn_t>
so that we are allowed to read the value while not holding
log_sys.mutex.

buf_flush_wait_flushed(): Let the page cleaner perform the flushing
also if innodb_flush_sync=OFF. After the page cleaner has
completed, perform a checkpoint if it is needed, because
buf_flush_sync_for_checkpoint() will not be run if
innodb_flush_sync=OFF.

buf_flush_ahead(): Simplify the condition. We do not really care
whether buf_flush_page_cleaner() is running.

buf_flush_page_cleaner(): Evaluate innodb_flush_sync at the low
level. If innodb_flush_sync=OFF, rate-limit the batches to
innodb_io_capacity_max pages per second.

Reviewed by: Vladislav Vaintroub