JounralingObjectStore: journal->committed_thru after replay

It's possible that the osd stopped between when the filestore
op_seq file was updated and when the journal was trimmed.  In
that case, it's possible that on boot the journal might be
full, and yet not be trimmed because commit_start assumes
there is no work to do.  Calling committed_thru on the journal
ensures that the journal matches committed_seq.

Backport: emperor dumpling
Fixes: 6756
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
JournalingObjectStore::journal_replay: don't call committed_thru

It suffices to set last_committed_seq correctly in
FileJournal::journal_replay.  This call to committed_thru can race with
the sync thread and trigger the assert(seq >= committed_seq) in
committed_thru.

Introduced in d8d27f13e11dcaefd3aa1c049b97c980283da575
Fixes: #6786
Signed-off-by: Samuel Just <sam.just@inktank.com>