MDEV-24834 Assertion mtr->memo_contains_flagged() in btr0cur.cc:1500

A too strict debug assertion was introduced in
commit 03ca6495 (MDEV-24142).
It turns out that row_ins_sec_index_entry_low() may acquire a stronger
latch on the index. The old rw_lock_own(..., RW_LOCK_S) assertion
would hold also for the SX (Update) latch mode.

btr_cur_search_to_nth_level_func(): Relax the assertion to require
that the mini-transaction hold any of S or U latch.

storage/innobase/btr/btr0cur.cc

@@ -1498,7 +1498,8 @@ btr_cur_search_to_nth_level_func(
 		if (!srv_read_only_mode) {
 			if (s_latch_by_caller) {
 				ut_ad(mtr->memo_contains_flagged(
-					      &index->lock, MTR_MEMO_S_LOCK));
+					      &index->lock, MTR_MEMO_S_LOCK
+					      | MTR_MEMO_SX_LOCK));
 			} else if (!modify_external) {
 				/* BTR_SEARCH_TREE is intended to be used with
 				BTR_ALREADY_S_LATCHED */