libceph: Allocate larger oid buffer in request msgs

ceph_osd_request struct allocates a 40-byte buffer for object names.
RBD image names can be up to 96 chars long (100 with the .rbd suffix),
which results in the object name for the image being truncated, and a
subsequent map failure.

Increase the oid buffer in request messages, in order to avoid the
truncation.

Signed-off-by: Stratos Psomadakis <psomas@grnet.gr>
Signed-off-by: Sage Weil <sage@newdream.net>