exportfs: use lookup_one_unlocked()

rather than locking the directory and using lookup_one(), just use
lookup_one_unlocked().  This keeps locking code centralised.

Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/20250608230952.20539-5-neil@brown.name


Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>