osdc/Journaler: fix try_read_entry()/wait_for_readable() locking

These two functions should be protected by the Journal::mutex. Furthermore,
wait_for_readable() is racy. The journal may have already become readable
when it is called.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #2404 from ceph/wip-9358

osdc/Journaler: fix try_read_entry()/wait_for_readable() locking

Reviewed-by: John Spray <john.spray@redhat.com>