Add 'zfs umount -u' for encrypted datasets

Add 'zfs umount -u' for encrypted datasets

This patch adds the ability for the user to unload keys for
datasets as they are being unmounted. This is analogous to
'zfs mount -l'.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes: #8917
Closes: #8952