ntfs3: serve as alias for the legacy ntfs driver

Johan Hovold reported that removing the legacy ntfs driver broke boot
for him since his fstab uses the legacy ntfs driver to access firmware
from the original Windows partition.

Use ntfs3 as an alias for legacy ntfs if CONFIG_NTFS_FS is selected.
This is similar to how ext3 is treated.

Link: https://lore.kernel.org/r/Zf2zPf5TO5oYt3I3@hovoldconsulting.com
Link: https://lore.kernel.org/r/20240325-hinkriegen-zuziehen-d7e2c490427a@brauner
Fixes: 7ffa8f3d ("fs: Remove NTFS classic")
Tested-by: Johan Hovold <johan+linaro@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/ntfs3/Kconfig

@@ -46,3 +46,12 @@ config NTFS3_FS_POSIX_ACL
 	  NOTE: this is linux only feature. Windows will ignore these ACLs.
 
 	  If you don't know what Access Control Lists are, say N.
+
+config NTFS_FS
+	tristate "NTFS file system support"
+	select NTFS3_FS
+	select BUFFER_HEAD
+	select NLS
+	help
+	  This config option is here only for backward compatibility. NTFS
+	  filesystem is now handled by the NTFS3 driver.

fs/ntfs3/super.c

@@ -1798,6 +1798,35 @@ static struct file_system_type ntfs_fs_type = {
 	.kill_sb		= ntfs3_kill_sb,
 	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
 };
+
+#if IS_ENABLED(CONFIG_NTFS_FS)
+static struct file_system_type ntfs_legacy_fs_type = {
+	.owner			= THIS_MODULE,
+	.name			= "ntfs",
+	.init_fs_context	= ntfs_init_fs_context,
+	.parameters		= ntfs_fs_parameters,
+	.kill_sb		= ntfs3_kill_sb,
+	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
+};
+MODULE_ALIAS_FS("ntfs");
+
+static inline void register_as_ntfs_legacy(void)
+{
+	int err = register_filesystem(&ntfs_legacy_fs_type);
+	if (err)
+		pr_warn("ntfs3: Failed to register legacy ntfs filesystem driver: %d\n", err);
+}
+
+static inline void unregister_as_ntfs_legacy(void)
+{
+	unregister_filesystem(&ntfs_legacy_fs_type);
+}
+#else
+static inline void register_as_ntfs_legacy(void) {}
+static inline void unregister_as_ntfs_legacy(void) {}
+#endif
+
+
 // clang-format on
 
 static int __init init_ntfs_fs(void)
@@ -1832,6 +1861,7 @@ static int __init init_ntfs_fs(void)
 		goto out1;
 	}
 
+	register_as_ntfs_legacy();
 	err = register_filesystem(&ntfs_fs_type);
 	if (err)
 		goto out;
@@ -1849,6 +1879,7 @@ static void __exit exit_ntfs_fs(void)
 	rcu_barrier();
 	kmem_cache_destroy(ntfs_inode_cachep);
 	unregister_filesystem(&ntfs_fs_type);
+	unregister_as_ntfs_legacy();
 	ntfs3_exit_bitmap();
 
 #ifdef CONFIG_PROC_FS