nfsd: Fix null-ptr-deref in nfsd_fill_super()

KASAN report null-ptr-deref as follows:

  BUG: KASAN: null-ptr-deref in nfsd_fill_super+0xc6/0xe0 [nfsd]
  Write of size 8 at addr 000000000000005d by task a.out/852

  CPU: 7 PID: 852 Comm: a.out Not tainted 5.18.0-rc7-dirty #66
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-1.fc33 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x34/0x44
   kasan_report+0xab/0x120
   ? nfsd_mkdir+0x71/0x1c0 [nfsd]
   ? nfsd_fill_super+0xc6/0xe0 [nfsd]
   nfsd_fill_super+0xc6/0xe0 [nfsd]
   ? nfsd_mkdir+0x1c0/0x1c0 [nfsd]
   get_tree_keyed+0x8e/0x100
   vfs_get_tree+0x41/0xf0
   __do_sys_fsconfig+0x590/0x670
   ? fscontext_read+0x180/0x180
   ? anon_inode_getfd+0x4f/0x70
   do_syscall_64+0x35/0x80
   entry_SYSCALL_64_after_hwframe+0x44/0xae

This can be reproduce by concurrent operations:
	1. fsopen(nfsd)/fsconfig
	2. insmod/rmmod nfsd

Since the nfsd file system is registered before than nfsd_net allocated,
the caller may get the file_system_type and use the nfsd_net before it
allocated, then null-ptr-deref occurred.

So init_nfsd() should call register_filesystem() last.

Fixes: bd5ae928 ("nfsd: register pernet ops last, unregister first")
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfsctl.c

@@ -1535,25 +1535,25 @@ static int __init init_nfsd(void)
 	retval = create_proc_exports_entry();
 	if (retval)
 		goto out_free_lockd;
-	retval = register_filesystem(&nfsd_fs_type);
-	if (retval)
-		goto out_free_exports;
 	retval = register_pernet_subsys(&nfsd_net_ops);
 	if (retval < 0)
-		goto out_free_filesystem;
+		goto out_free_exports;
 	retval = register_cld_notifier();
 	if (retval)
 		goto out_free_subsys;
 	retval = nfsd4_create_laundry_wq();
+	if (retval)
+		goto out_free_cld;
+	retval = register_filesystem(&nfsd_fs_type);
 	if (retval)
 		goto out_free_all;
 	return 0;
 out_free_all:
+	nfsd4_destroy_laundry_wq();
+out_free_cld:
 	unregister_cld_notifier();
 out_free_subsys:
 	unregister_pernet_subsys(&nfsd_net_ops);
-out_free_filesystem:
-	unregister_filesystem(&nfsd_fs_type);
 out_free_exports:
 	remove_proc_entry("fs/nfs/exports", NULL);
 	remove_proc_entry("fs/nfs", NULL);
@@ -1571,6 +1571,7 @@ static int __init init_nfsd(void)
 
 static void __exit exit_nfsd(void)
 {
+	unregister_filesystem(&nfsd_fs_type);
 	nfsd4_destroy_laundry_wq();
 	unregister_cld_notifier();
 	unregister_pernet_subsys(&nfsd_net_ops);
@@ -1581,7 +1582,6 @@ static void __exit exit_nfsd(void)
 	nfsd_lockd_shutdown();
 	nfsd4_free_slabs();
 	nfsd4_exit_pnfs();
-	unregister_filesystem(&nfsd_fs_type);
 }
 
 MODULE_AUTHOR("Olaf Kirch <okir@monad.swb.de>");