PG::init: clear rollback info for backfill as well

Otherwise, we won't remove the old rollback objects from a resurrected pg.  In
rare cases, this can cause us to get an EEXIST if we happen to reuse the same
rename id on the same object in a subsequent interval.

Fixes: #9293
Related to: 8346e10755027e982f26bab4642334fd91cc31aa
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>