PGLog::claim_log_and_clear_rollback_info: fix rollback_info_trimmed_to

We have been setting it to the old head value.  This is usually
harmless since the new head will virtually always be ahead of the
old head for claim_log_and_clear_rollback_info, but can cause trouble
in some edge cases.

Fixes: #9481
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>