fs: indicate request originates from old mount API

We already communicate to filesystems when a remount request comes from
the old mount API as some filesystems choose to implement different
behavior in the new mount API than the old mount API to e.g., take the
chance to fix significant API bugs. Allow the same for regular mount
requests.

Fixes: b330966f ("fuse: reject options on reconfigure via fsconfig(2)")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/namespace.c

@@ -2875,7 +2875,12 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 	if (IS_ERR(fc))
 		return PTR_ERR(fc);
 
+	/*
+	 * Indicate to the filesystem that the remount request is coming
+	 * from the legacy mount system call.
+	 */
 	fc->oldapi = true;
+
 	err = parse_monolithic_mount_data(fc, data);
 	if (!err) {
 		down_write(&sb->s_umount);
@@ -3324,6 +3329,12 @@ static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
 	if (IS_ERR(fc))
 		return PTR_ERR(fc);
 
+	/*
+	 * Indicate to the filesystem that the mount request is coming
+	 * from the legacy mount system call.
+	 */
+	fc->oldapi = true;
+
 	if (subtype)
 		err = vfs_parse_fs_string(fc, "subtype",
 					  subtype, strlen(subtype));