Fix uninitialized value read in vdev_prop_set()

If no errors are encountered, we read an uninitialized error value.

Clang's static analyzer complained about this.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14007