selinux: simplify avtab_insert_node() prototype

__hashtab_insert() in hashtab.h has a cleaner interface that allows the
caller to specify the chain node location that the new node is being
inserted into so that it can update the node that currently occupies it.

Signed-off-by: Jacob Satterfield <jsatterfield.linux@gmail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>