ZTS: Fix and reenable zfs_rename tests

zfs_rename_006_pos has been flaky in the past because it was
missing a call to block_device_wait to ensure the zvols it creates
are present before running dd. Whenever this this happened,
zfs_rename_009_neg would also fail because the first test would
leak a zvol clone that it did not know how to clean up. This patch
fixes the root cause and reenables the test. It also fixes some
minor grammar errors.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #5647
Closes #5648
Closes #8088