ceph: Fix return value of encode_fh function

encode_fh function should return 255 on error as done by other file
system to indicate EOVERFLOW. Also max_len is in sizeof(u32) units
and not in bytes.

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