ovl: narrow locking in ovl_clear_empty()

ovl: narrow locking in ovl_clear_empty()

Drop the locks immediately after rename, and use a separate lock for
cleanup.

This makes way for future changes where locks are taken on individual
dentries rather than the whole directory.

Note that ovl_cleanup_whiteouts() operates on "upper", a child of
"upperdir" and does not require upperdir or workdir to be locked.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/20250716004725.1206467-7-neil@brown.name
Signed-off-by: Christian Brauner <brauner@kernel.org>