ovl: pass string to ovl_parse_layer()

[ Upstream commit 7eff3453cbd7e0bfc7524d59694119b5ca844778 ]

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>
Stable-dep-of: ca76ac36bb60 ("ovl: fix wrong lowerdir number check for parameter Opt_lowerdir")
Signed-off-by: Sasha Levin <sashal@kernel.org>