Fix zfs_ioc_pool_sync should not use fnvlist

Use fnvlist on user input would allow user to easily panic zfs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #6529