Add '\n' for pr_err in function ovl_parse_param_lowerdir(), which
ensures that error message is displayed at once.

Fixes: b36a5780 ("ovl: modify layer parameter parsing")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Link: https://lore.kernel.org/r/20240705011510.794025-4-chengzhihao1@huawei.comSigned-off-by: Christian Brauner <brauner@kernel.org>