Initialize parity blocks before RAID-Z reconstruction benchmarking

benchmark_raidz() allocates a row to benchmark parity calculation and
reconstruction. In the latter case, the parity blocks are left
uninitialized, leading to reports from KMSAN.

Initialize parity blocks to 0xAA as we do for the data earlier in the
function. This does not affect the selected RAID-Z implementation on
any of several systems tested.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #12473