Fix ZTS grow_pool/setup

The addition of the large_dnode_008_pos test case, which runs
right before this one, exposed some racy behavior in grow_pool
setup.sh on the Ubuntu kmemleak builder. Before creating
partitions on a device destroying any existing ones.

  ERROR: set_partition 1  100mb loop0 exited 1

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6499
Closes #6516