NFSD: Add /sys/kernel/debug/nfsd

Create a small sandbox under /sys/kernel/debug for experimental NFS
server feature settings. There is no API/ABI compatibility guarantee
for these settings.

The only documentation for such settings, if any documentation exists,
is in the kernel source code.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>