subr_pctrie: improve iter nbr search

[dougm]

Summary

pctrie_toval(node) can be applied to either a leaf or an internal node; in the latter case it provides the address of the pn_owner field. In a couple of places where a neighbor search is about to begin for an iterator, the current code distinguishes the leaf and non-leaf cases in a way that isn't really necessary. This change shrinks each function by 16 bytes, and by a branch instruction.