Use zfs_ioctl with zfs_cmd_t in libzfs

Use zfs_ioctl with zfs_cmd_t in libzfs

Consistently use the `zfs_ioctl()` wrapper since `ioctl()` cannot be
called directly due to differing semantics between platforms.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9492