libceph: provide data length when preparing message

In prepare_message_data(), the length used to initialize the cursor
is taken from the header of the message provided.  I'm working
toward not using the header data length field to determine length in
outbound messages, and this is a step in that direction.  For
inbound messages this will be set to be the actual number of bytes
that are arriving (which may be less than the total size of the data
buffer available).

This resolves:
    http://tracker.ceph.com/issues/4589



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>