coda: use iterate_dir() in coda_readdir()

coda: use iterate_dir() in coda_readdir()

The code in coda_readdir() is nearly identical to iterate_dir().
Differences are:
 - iterate_dir() is killable
 - iterate_dir() adds permission checking and accessing notifications

I believe these are not harmful for coda so it is best to use
iterate_dir() directly.  This will allow locking changes without
touching the code in coda.

Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/20250608230952.20539-4-neil@brown.name
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>