Fix encryption root hierarchy issue

After doing a recursive raw receive, zfs userspace performs
a final pass to adjust the encryption root hierarchy as
needed. Unfortunately, the FORCE_INHERIT ioctl had a bug
which caused the encryption root to always be assigned to
the direct parent instead of the inheriting parent. This
patch simply fixes this issue.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #6847
Closes #6848