pctrie: unlock node store in remove

In pctrie_remove(), if the removal of an item leaves an internal node
with one child, then there are two calls to pctrie_node_store(). Only
the second of these two needs to be stored with PCTRIE_LOCKED
synchronization. Eliminate pointless synchronization for the first
node_store in that case.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49033