ceph: Update max_len with minimum required size

encode_fh on error should update max_len with minimum required
size, so that caller can redo the call with the reallocated buffer.
This is required with open by handle patch series

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Sage Weil <sage@newdream.net>