ceph: only let auth caps update max_size

Only the auth MDS has a meaningful max_size value for us, so only update it
in fill_inode if we're being issued an auth cap.  Otherwise, a random
stat result from a non-auth MDS can clobber a meaningful max_size, get
the client<->mds cap state out of sync, and make writes hang.

Specifically, even if the client re-requests a larger max_size (which it
will), the MDS won't respond because as far as it knows we already have a
sufficiently large value.

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