sunrpc: add a debugfs rpc_xprt directory with an info file in it

Add a new directory heirarchy under the debugfs sunrpc/ directory:

    sunrpc/
        rpc_xprt/
            <xprt id>/

Within that directory, we can put files that give info about the
xprts. We do have the (minor) problem that there is no succinct,
unique identifier for rpc_xprts. So we generate them synthetically
with a static atomic_t counter.

For now, this directory just holds an "info" file, but we may add
other files to it in the future.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
index cf391ee..9d27ac4 100644
--- a/include/linux/sunrpc/xprt.h
+++ b/include/linux/sunrpc/xprt.h
@@ -239,6 +239,9 @@
 	struct net		*xprt_net;
 	const char		*servername;
 	const char		*address_strings[RPC_DISPLAY_MAX];
+#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
+	struct dentry		*debugfs;		/* debugfs directory */
+#endif
 };
 
 #if defined(CONFIG_SUNRPC_BACKCHANNEL)