pctrie: correct iter node after node allocation

Usually, a pctrie_iter operation leaves the iterator node pointing to
the last pctrie_node on the path to the found, or inserted, or removed
leaf. However, that is not the case when inserting a leaf causes the
allocation of a new pctrie node. In that case, the iterator node is
left pointing to the parent of the allocated node, and not to the
newly allocated node itself, the parent of the just-inserted
leaf. Correct this anomaly.

Reviewed by: markj, kib, alc
Differential Revision: https://reviews.freebsd.org/D51059