mds: fix null snapflush logic
mds: fix null snapflush logic

We only want to do a null snapflush if we _know_ there isn't another one
coming: that is, there aren't any outstanding issued excl/wr cap bits at
the client.  The old test has the bitwise NOT backwards.  We can also
limit the test to the bits we care about.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph