ovl: pass string to ovl_parse_layer()

So it can be used for parsing the Opt_lowerdir.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Link: https://lore.kernel.org/r/20240705011510.794025-2-chengzhihao1@huawei.com


Signed-off-by: Christian Brauner <brauner@kernel.org>