ovl: narrow locking in ovl_check_rename_whiteout()

ovl: narrow locking in ovl_check_rename_whiteout()

ovl_check_rename_whiteout() now only holds the directory lock when
needed, and takes it again if necessary.

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/20250716004725.1206467-20-neil@brown.name
Signed-off-by: Christian Brauner <brauner@kernel.org>