nilfs2: Fix return in nilfs_mkdir

Return NULL instead of passing zero to ERR_PTR.
  Fixes smatch warning:
     - fs/nilfs2/namei.c:261 nilfs_mkdir() warn: passing zero to 'ERR_PTR'

Fixes: 88d5baf ("Change inode_operations.mkdir to return struct dentry *")
Signed-off-by: Hongling Zeng <zenghongling@kylinos.cn>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>

Author: zenghongling0719

fs/nilfs2/namei.c

@@ -258,7 +258,7 @@ static struct dentry *nilfs_mkdir(struct mnt_idmap *idmap, struct inode *dir,
 	else
 		nilfs_transaction_abort(dir->i_sb);
 
-	return ERR_PTR(err);
+	return err ? ERR_PTR(err) : NULL;
 
 out_fail:
 	drop_nlink(inode);