os/bluestore/BlueFS: fix race with log flush during async log compaction

os/bluestore/BlueFS: fix race with log flush during async log compaction

During async log compaction we rely on _flush-and_sync_log to update the
log_writer to jump_to.  However, if racing threads are also trying to flush
the log and manage to flush our new log events for us, then our flush will
turn into a no-op, and we won't update jump_to correctly at all.  This
results in a corrupted log size a bit later one.

Fix by ensuring that there are no in-progress flushes before we add our
log entries.  Also, add asserts to _flush_and_sync_log to make sure we
never bail out early if jump_to is set (which would indicate this or
another similar bug is still present).

Fixes: http://tracker.ceph.com/issues/21878
Signed-off-by: Sage Weil <sage@redhat.com>