Write label 2,3 uberblocks when vdev expands

When vdev_psize increases, the location of labels 2 and 3 changes
because their location is relative to the end of the device.

The configs for labels 2 and 3 are written during the next spa_sync()
because the vdev is added to the dirty config list. However, the
uberblock rings are not re-written in their new location, leaving the
device vulnerable to the beginning of the device being overwritten or
damaged.

This patch copies the uberblock ring from label 0 to labels 2 and 3,
in their new locations, at the next sync after vdev_psize increases.

Also, add a test zpool_expand_004_pos.ksh to confirm the uberblocks
are copied.

Reviewed-by: BearBabyLiu <liu.huang@zte.com.cn>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #5108