Only check the blessings table for known LORs.

Previously we would check for blessings before marking a given lock
pair as reversed, so each "reversed" lock acquisition would require
a linear scan of the table. Instead, check the table after marking
the pair as reversed but before generating a report.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21135