Fix optional "force" arg handing in zfs_ioc_pool_sync()

The fnvlist_lookup_boolean_value() function should not be used
to check the force argument since it's optional. It may not be
provided or may have been created with the wrong flags.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11281
Closes #11284