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 <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/Makefile

···
 nfsd-$(CONFIG_NFSD_SCSILAYOUT) += blocklayout.o blocklayoutxdr.o
 nfsd-$(CONFIG_NFSD_FLEXFILELAYOUT) += flexfilelayout.o flexfilelayoutxdr.o
 nfsd-$(CONFIG_NFS_LOCALIO) += localio.o
+nfsd-$(CONFIG_DEBUG_FS) += debugfs.o
 
 
 .PHONY: xdrgen

fs/nfsd/debugfs.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/debugfs.h>
+
+#include "nfsd.h"
+
+static struct dentry *nfsd_top_dir __read_mostly;
+
+void nfsd_debugfs_exit(void)
+{
+	debugfs_remove_recursive(nfsd_top_dir);
+	nfsd_top_dir = NULL;
+}
+
+void nfsd_debugfs_init(void)
+{
+	nfsd_top_dir = debugfs_create_dir("nfsd", NULL);
+}

fs/nfsd/nfsctl.c

···
 {
 	int retval;
 
+	nfsd_debugfs_init();
+
 	retval = nfsd4_init_slabs();
 	if (retval)
 		return retval;
···
 	nfsd4_exit_pnfs();
 out_free_slabs:
 	nfsd4_free_slabs();
+	nfsd_debugfs_exit();
 	return retval;
 }
 
···
 	nfsd_lockd_shutdown();
 	nfsd4_free_slabs();
 	nfsd4_exit_pnfs();
+	nfsd_debugfs_exit();
 }
 
 MODULE_AUTHOR("Olaf Kirch <[e-mail]>");

fs/nfsd/nfsd.h

···
 int nfsd_create_serv(struct net *net);
 void nfsd_destroy_serv(struct net *net);
 
+#ifdef CONFIG_DEBUG_FS
+void nfsd_debugfs_init(void);
+void nfsd_debugfs_exit(void);
+#else
+static inline void nfsd_debugfs_init(void) {}
+static inline void nfsd_debugfs_exit(void) {}
+#endif
+
 extern int nfsd_max_blksize;
 
 static inline int nfsd_v4client(struct svc_rqst *rq)