Use zfs_ioctl() in zinject.c

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

Follow up PR to #9492.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9507