fix a serious bug in r258632: offset parameter must be set in zio

In illumos all ioctl zio-s are "global" at the moment. That is they act
on a whole disk, e.g. a cache flush command, and thus do not need either
offset or size parameters.
FreeBSD, on the other hand, has support for TRIM command and that
command requires proper offset and size parameters.
Without this fix all TRIM commands act on the start of any disk or
partition used by ZFS destroying any data there.

Pointyhat to: avg
Tested by: sbruno
MFC after: 3 days
X-MFC with: r258632
Sponsored by: HybridCluster