client: _flush should no-op if nothing to flush
client: _flush should no-op if nothing to flush

If there are no FILE_BUFFER cap_refs, then we can bail out early.
Otherwise we will end up dropping refs we don't have.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

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