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

In commit 7cffb5f6 (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.