ZTS: Fix create-o_ashift test case

The function that fills the uberblock ring buffer on every device label
has been reworked to avoid occasional failures caused by a race
condition that prevents 'zpool sync' from writing some uberblock
sequentially: this happens when the pool sync ioctl dispatch code calls
txg_wait_synced() while we're already waiting for a TXG to sync.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6924
Closes #6977