MDEV-25072: Livelock due to innodb_change_buffering_debug

buf_page_get_low(): Do not try to re-evict the page if it is
multiply buffer-fixed.

In commit 7cffb5f (MDEV-23399)
a livelock was introduced. If multiple threads are concurrently
requesting the same secondary index leaf page in buf_page_get_low()
and innodb_change_buffering_debug is set, all threads would try
to evict the page in a busy loop, never succeeding because the
block is buffer-fixed by other threads.

Thanks to Roel Van de Paar for reporting the original failure and
Elena Stepanova for producing an "rr replay" trace.

storage/innobase/buf/buf0buf.cc

@@ -3324,7 +3324,8 @@ buf_page_get_low(
 		buf_flush_lists(ULINT_UNDEFINED, LSN_MAX);
 		buf_flush_wait_batch_end_acquiring_mutex(false);
 
-		if (!fix_block->page.oldest_modification()) {
+		if (fix_block->page.buf_fix_count() == 1
+		    && !fix_block->page.oldest_modification()) {
 			goto re_evict;
 		}