subr_pctrie: improve iter nbr search

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.

Reviewed by: bnovkov
Differential Revision: https://reviews.freebsd.org/D47207