MDEV-8390: innodb.innodb-flush-changed-page-bitmaps crashes

Analysis: Flush changed page bitmaps calls log_online_follow_redo_log
that later calls log_group_read_log_seg with release_mutex = true
and that causes the log_sys mutex being released before
log_decrypt_after_read function that later required log_sys mutex.

Fix: Enter log_sys mutex before log_decrypt_after_read if mutex
is released and release the mutex after the function if we
acquired it before function call.

storage/xtradb/log/log0log.cc

@@ -2581,8 +2581,16 @@ loop:
 	       (ulint) (source_offset % UNIV_PAGE_SIZE),
 	       len, buf, (type == LOG_ARCHIVE) ? &log_archive_io : NULL, 0);
 
+	if (release_mutex) {
+		mutex_enter(&log_sys->mutex);
+	}
+
 	log_decrypt_after_read(buf, len);
 
+	if (release_mutex) {
+		mutex_exit(&log_sys->mutex);
+	}
+
 	start_lsn += len;
 	buf += len;