MDEV-31816 fixup: Relax a debug assertion

buf_LRU_free_page(): The block may also be in the IBUF_EXIST state
when executing the test innodb.innodb_bulk_create_index_debug.

storage/innobase/buf/buf0lru.cc

@@ -855,6 +855,7 @@ bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
 			ut_d(uint32_t s=) b->fix();
 			ut_ad(s == buf_page_t::FREED
 			      || s == buf_page_t::UNFIXED
+			      || s == buf_page_t::IBUF_EXIST
 			      || s == buf_page_t::REINIT);
 		}
 		break;