bugzilla: https://bugzilla.openanolis.cn/show_bug.cgi?id=1849

The following patches expose information about NFSv4 state held by knfsd
on behalf of NFSv4 clients. This is especially important for opens,
which are currently invisible to userspace on the server, unlike locks
(/proc/locks) and local processes' opens (under /proc//).
The approach is to add a new directory /proc/fs/nfsd/clients/ with
subdirectories for each active NFSv4 client. Each subdirectory has an
"info" file with some basic information to help identify the client and
a "states" directory that lists the open state held by that client.