SUNRPC: Remove warning in debugfs.c when compiling with W=1

Remove the following warning:

net/sunrpc/debugfs.c:13: warning: cannot understand function prototype: 'struct dentry *topdir;
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/debugfs.c

 // SPDX-License-Identifier: GPL-2.0
-/**
+/*
  * debugfs interface for sunrpc
  *
  * (c) 2014 Jeff Layton <jlayton@primarydata.com>