riscv: fix pv_chunk check in pmap_pv_demote_l2()

It is the full chunk that should be placed at the tail of the list, not
the free chunk.

Reviewed by: mhorne, jhb
MFC after: 1 week3 days
Fixes: 4d90a5afc51b ("sys: Consolidate common implementation details...")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1321