Make 'zpool labelclear -f' work on offlined disks

This patch allows you to clear the label on offlined disks in an active
pool with -f. Previously, labelclear wouldn't let you do that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #12511