Fix regression due to "fs: move binfmt_misc sysctl to its own file"

Commit 3ba442d5 ("fs: move binfmt_misc sysctl to its own file") did
not go unnoticed, binfmt-support stopped to work on my Debian system
since v5.17-rc2 (did not check with -rc1).

The existance of the /proc/sys/fs/binfmt_misc is a precondition for
attempting to mount the binfmt_misc fs, which in turn triggers the
autoload of the binfmt_misc module.  Without it, no module is loaded and
no binfmt is available at boot.

Building as built-in or manually loading the module and mounting the fs
works fine, it's therefore only a matter of interaction with user-space.
I could try to improve the Debian systemd configuration but I can't say
anything about the other distributions.

This patch restores a working system right after boot.

Fixes: 3ba442d5 ("fs: move binfmt_misc sysctl to its own file")
Signed-off-by: NDomenico Andreoli <domenico.andreoli@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: NTong Zhang <ztong0001@gmail.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_misc.c

@@ -817,20 +817,16 @@ static struct file_system_type bm_fs_type = {
 };
 MODULE_ALIAS_FS("binfmt_misc");
 
-static struct ctl_table_header *binfmt_misc_header;
-
 static int __init init_misc_binfmt(void)
 {
 	int err = register_filesystem(&bm_fs_type);
 	if (!err)
 		insert_binfmt(&misc_format);
-	binfmt_misc_header = register_sysctl_mount_point("fs/binfmt_misc");
-	return 0;
+	return err;
 }
 
 static void __exit exit_misc_binfmt(void)
 {
-	unregister_sysctl_table(binfmt_misc_header);
 	unregister_binfmt(&misc_format);
 	unregister_filesystem(&bm_fs_type);
 }

fs/file_table.c

@@ -119,6 +119,8 @@ static struct ctl_table fs_stat_sysctls[] = {
 static int __init init_fs_stat_sysctls(void)
 {
 	register_sysctl_init("fs", fs_stat_sysctls);
+	if (IS_ENABLED(CONFIG_BINFMT_MISC))
+		register_sysctl_mount_point("fs/binfmt_misc");
 	return 0;
 }
 fs_initcall(init_fs_stat_sysctls);