Fix reference counting in pmap_ts_referenced() on RISC-V.

pmap_ts_referenced() does not necessarily clear the access bit from
all accessed mappings of a given page. Thus, if a scan of the mappings
needs to be restarted, we should be careful to avoid double-counting
accessed mappings whose access bits were not cleared in a previous
attempt.

Reported by: alc
Reviewed by: alc
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20926