libzfs_core: Initialize fail_ioc_cmd to ZFS_IOC_LAST

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