ovl: narrow locking in ovl_workdir_create()

In ovl_workdir_create() don't hold the dir lock for the whole time, but
only take it when needed.

It now gets taken separately for ovl_workdir_cleanup().  A subsequent
patch will move the locking into that function.

Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/20250716004725.1206467-13-neil@brown.name


Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>