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: NSage Weil <sage@newdream.net>