ovl: narrow locking in ovl_create_upper()

Drop the directory lock immediately after the ovl_create_real() call and
take a separate lock later for cleanup in ovl_cleanup_unlocked() - if
needed.

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

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/20250716004725.1206467-6-neil@brown.name


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