ovl: properly print correct variable

ovl: properly print correct variable

In case of ovl_lookup_temp() failure, we currently print `err`
which is actually not initialized at all.

Instead, properly print PTR_ERR(whiteout) which is where the
actual error really is.

Address-Coverity-ID: 1647983 ("Uninitialized variables  (UNINIT)")
Fixes: 8afa0a7367138 ("ovl: narrow locking in ovl_whiteout()")
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
Link: https://lore.kernel.org/20250721203821.7812-1-antonio@mandelbit.com
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Christian Brauner <brauner@kernel.org>