sftp: fix directory creation races

If mkdir fails then before this change it would have thrown an
error.

After this change, if the error indicated that the directory
already exists then the error is not returned to the user.

This fixes a race condition when two rclone threads are trying to
create the same directory.

backend/sftp/sftp.go

@@ -1171,6 +1171,10 @@ func (f *Fs) mkdir(ctx context.Context, dirPath string) error {
 	err = c.sftpClient.Mkdir(dirPath)
 	f.putSftpConnection(&c, err)
 	if err != nil {
+		if os.IsExist(err) {
+			fs.Debugf(f, "directory %q exists after Mkdir is attempted", dirPath)
+			return nil
+		}
 		return fmt.Errorf("mkdir %q failed: %w", dirPath, err)
 	}
 	return nil