ceph: fix open for write on clustered mds

Normally when we open a file we already have a cap, and simply update the
wanted set.  However, if we open a file for write, but don't have an auth
cap, that doesn't work; we need to open a new cap with the auth MDS.  Only
reuse existing caps if we are opening for read or the existing cap is auth.

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