client: flush the mdlog in _fsync before waiting on unstable reqs

client: flush the mdlog in _fsync before waiting on unstable reqs

Recently, we changed ganesha to call to ceph_ll_sync_inode after
issuing a setattr call, to ensure that the attribute changes makes it
to the backing store before returning.

While testing with this, I noticed that we'd occasionally see setattr
calls stall for a long time. Analysis showed that the client's _fsync
request was stalled waiting on an mdlog flush.

This patch adds a call to flush_mdlog_sync prior to waiting on unsafe
requests in _fsync and seems to resolve the issue.

Tracker: http://tracker.ceph.com/issues/23714
Signed-off-by: Jeff Layton <jlayton@redhat.com>
(cherry picked from commit d790108158d21616fdf6ea148bf7f9d1534326d0)