Parallelize vdev_validate

The runtime of vdev_validate is dominated by the disk accesses in
vdev_label_read_config. Speed it up by validating all vdevs in
parallel using a taskq.

Sponsored by: Axcient
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #11470