bcachefs: Add missing bch2_err_class() call

We're not supposed to return our private error codes to userspace.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -1894,7 +1894,7 @@ static struct dentry *bch2_mount(struct file_system_type *fs_type,
 
 err_put_super:
 	deactivate_locked_super(sb);
-	return ERR_PTR(ret);
+	return ERR_PTR(bch2_err_class(ret));
 }
 
 static void bch2_kill_sb(struct super_block *sb)