os/FileStore::sync_entry check for stop in after wait

os/FileStore::sync_entry check for stop in after wait

there is chance that the sync_entry() gets signaled in the
WaitInterval(max_interval) call because of sync_cond.Signal() call after
"stop = true" in umount(), so without this change, sync_entry() will
continue wait until min_interval is reached. this forces umount() to
wait even it has called d_force_sync(), and hence slows down the
progress of umount(). so we need to check for `stop` if we are not
signalled because of `force_sync`.

Fixes: http://tracker.ceph.com/issues/15695
Reported-by: Haomai Wang <haomaiwang@gmail.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>