Fix pointer-is-uint64_t-sized assumption in the ioctl path

This shows up when compiling freebsd-head on amd64 using gcc-6.4.
The lib32 compat build ends up tripping over this assumption.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: adrian chadd <adrian@freebsd.org>
Closes #11068
Closes #11069