docs/zh_CN: add infiniband user_mad translation

Translate Documentation/infiniband/user_mad.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Puyu Wang <realpuyuwang@gmail.com>
Link: https://lore.kernel.org/r/8859ef2b40b380d7db7548f2e6a2d6fd5397062d.1628218477.git.siyanteng@loongson.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>