linuxkpi: remove invalid KASSERT from hash_add_rcu

hash_add_rcu asserted that the node's prev pointer was NULL in an
attempt to detect addition of a node already on a list, but the caller
is not required to provide a zeroed node.

Reported in https://github.com/freebsd/drm-kmod/issues/282

Reviewed by: bz, manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43645