bcachefs: Fix 'journal not marked as containing replicas'

bcachefs: Fix 'journal not marked as containing replicas'

This fixes the replicas_write_errors test: the patch
  bcachefs: mark journal replicas before journal write submission

partially fixed replicas marking for the journal, but it broke the case
where one replica failed - this patch re-adds marking after the journal
write completes, when we know how many replicas succeeded.

Additionally, we do not consider it a fsck error when the very last
journal entry is not correctly marked, since there is an inherent race
there.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>