devctl: Add missing validation to DEV_RESET

As in other ioctls which access the parent bus, we need to check for a
NULL parent here. Otherwise it's possible to trigger a null pointer
dereference by resetting the root device.

Reported by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D46965