docs/zh_CN: add infiniband user_verbs translation

Translate Documentation/infiniband/user_verbs.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/b7c1577cf9758943bff933c46200c7dff1e1c6e0.1628218477.git.siyanteng@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>