ceph: set osd request message front length correctly

We didn't set the front length correctly.  When messages used
the message pool we ended up with the conservative max (4 KB), and
the rest of the time the slightly less conservative estimate.  Even
though the OSD ignores the extra data, set it to the right value to avoid
sending extra data over the network.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>