pctrie: breakup pctrie_root_store

Break up pctrie_root_store into the part that casts the root to a
smr_pctnode_t *, and the rest. The rest is just pctrie_node_store, and
the casting part can be used in a few more places.

This is strictly a code-cleanup change, with no functional change
expected.

Reviewed by: bnovkov
Differential Revision: https://reviews.freebsd.org/D47347