osd: add flush_journal admin socket command

Add flush_journal admin socket command to be able to flush journal to
the permanent store for online osds.  (For offline osds we already have
ceph-osd --flush-journal.)

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
qa: test_alloc_hint: flush journal before prodding the FS

OSDs that for some reason get behind on processing their op queue break
expect_alloc_hint_eq(), as it pokes the FS and not the journal.  Fix it
by flushing the journal before proceeding with anything else.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1502 from ceph/wip-flush-journal-asok

wip-flush-journal-asok

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>