spa_prop_get: require caller to supply output nvlist

All callers to spa_prop_get() and spa_prop_get_nvlist() supplied their
own preallocated nvlist (except ztest), so we can remove the option to
have them allocate one if none is supplied.

This sidesteps a bug in spa_prop_get(), where the error var wasn't
initialised, which could lead to the provided nvlist being freed at the
end.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
(cherry picked from commit 366d6fecf6bb3c59668b0f3b89f2a610595f3d2f)