ceph: Check for err on mds request in atomic_open

The error returned by ceph_mdsc_do_request includes errors sending the
request, errors on timeout, or any errors coming from the mds.  If
ceph_mdsc_do_request returns an error, the reply struct will most likely
be bogus.  We need to bail out and propogate the error instead of
overwriting it.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>