mq: change mqueue msg mail to dynamic array

[zyfeier]

Summary

Change mqueue_msg_s to a dynamic array so that mqueue msg can be malloc based on the msg size of the current data when applied dynamically.

build base on https://github.com/apache/nuttx/pull/13281

Impact

NA

Testing

Test config: boards/arm/imx6/sabre-6quad/configs/nsh

Test case: ostest pass