MDEV-31120 Duplicate entry allowed into a UNIQUE column

row_ins_sec_index_entry_low(): Correct a condition that was
inadvertently inverted
in commit 89ec4b53 (MDEV-29603).

We are not supposed to buffer INSERT operations into unique indexes,
because duplicate key values would not be checked for. It is only
allowed when using unique_checks=0, and in that case the user is
supposed to guarantee that there are no duplicates.

storage/innobase/row/row0ins.cc

@@ -3005,8 +3005,8 @@ row_ins_sec_index_entry_low(
 			search_mode = btr_latch_mode(
 				search_mode
 				| (thr_get_trx(thr)->check_unique_secondary
-				   ? BTR_INSERT | BTR_IGNORE_SEC_UNIQUE
-				   : BTR_INSERT));
+				   ? BTR_INSERT
+				   : BTR_INSERT | BTR_IGNORE_SEC_UNIQUE));
 		}
 
 		err = cursor.search_leaf(entry, PAGE_CUR_LE, search_mode,