Micro-optimize dsl_prop_get_dd()

Use the saved property index instead of looking it up once per DSL
directory when traversing up towards the root.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Akash B <akash-b@hpe.com>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Sponsored-by: The FreeBSD Foundation
Closes #14397