PS-10232 [8.0]: Fix performance regression by limiting rows_in_buffer to query LIMIT

A follow up fix (2969af5) for "Bug#36775910: Record buffer not set in
index range scans" caused a performance regression by failing to
respect the handler's recommendation to cap on number of rows in
the record buffer (at the time of writing hard-coded to 100 for
InnoDB). This caused a performance regression which is most pronounced
when using a small LIMIT.

Fixed by re-introducing the cap.

Future work: As a future optimization, it would likely be beneficial
to communicate the LIMIT value to the executor so that it could
reduce the buffer size to that value - or not allocate a buffer at
all in case of LIMIT 1.

Author: inikep

sql/sql_executor.cc

@@ -739,6 +739,9 @@ bool set_record_buffer(TABLE *table, double expected_rows_to_fetch) {
       const ha_rows local_max_rows = (MAX_RECORD_BUFFER_SIZE / record_size);
       rows_in_buffer = std::clamp(rows_in_buffer, min_rows, local_max_rows);
     }
+
+    // Cap the number of rows according to handler's wish.
+    rows_in_buffer = std::min(rows_in_buffer, max_rows);
   }
 
   // After adjustments made above, we still need a minimum of 2 rows to