doc: zh_CN: add translatation for tmpfs

Translate Documentation/filesystems/tmpfs.rst into Chinese.

Signed-off-by: Wang Qing <wangqing@vivo.com>

Changes in v4:
 - Modify as Alex required.

Changes in v3:
 - Fix patch format issue.

Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: https://lore.kernel.org/r/1604887072-12997-1-git-send-email-wangqing@vivo.com


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