MDEV-26811: Assertion "log_sys->n_pending_flushes == 1" fails

In commit 1cb218c (MDEV-26450)
we introduced the function log_write_and_flush(), which may
compete with log_checkpoint() invoking log_write_flush_to_disk_low()
from another thread.

The assertion n_pending_flushes==1 is too strict.
There is no possibility of a race condition here, because
fil_flush() is protected by fil_system->mutex and the
rest will be protected by log_sys->mutex.

log_write_flush_to_disk_low(), log_write_and_flush():
Relax the assertions to test for a nonzero count.

Author: dr-m

storage/innobase/log/log0log.cc

@@ -983,21 +983,16 @@ log_group_write_buf(
 
 /** Flush the recently written changes to the log file.
 and invoke log_mutex_enter(). */
-static
-void
-log_write_flush_to_disk_low()
+static void log_write_flush_to_disk_low()
 {
-	/* FIXME: This is not holding log_sys->mutex while
-	calling os_event_set()! */
-	ut_a(log_sys->n_pending_flushes == 1); /* No other threads here */
+	ut_a(log_sys->n_pending_flushes);
 
 	bool	do_flush = srv_file_flush_method != SRV_O_DSYNC;
 
 	if (do_flush) {
 		fil_flush(SRV_LOG_SPACE_FIRST_ID);
 	}
 
-
 	log_mutex_enter();
 	if (do_flush) {
 		log_sys->flushed_to_disk_lsn = log_sys->current_flush_lsn;
@@ -1329,7 +1324,7 @@ ATTRIBUTE_COLD void log_write_and_flush()
 
   /* Code adapted from log_write_flush_to_disk_low() */
 
-  ut_a(log_sys->n_pending_flushes == 1); /* No other threads here */
+  ut_a(log_sys->n_pending_flushes);
 
   if (srv_file_flush_method != SRV_O_DSYNC)
     fil_flush(SRV_LOG_SPACE_FIRST_ID);