FreeBSD numbers `ZFS_IOC_*` starting at 0, so pick a different
sentinel value to avoid unintentionally messing with
`ZFS_IOC_POOL_CREATE` ioctls.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes#10729