libceph: record length of bio list with bio

When assigning a bio pointer to an osd request, we don't have an
efficient way of knowing the total length bytes in the bio list.
That information is available at the point it's set up by the rbd
code, so record it with the osd data when it's set.

This and the next patch are related to maintaining the length of a
message's data independent of the message header, as described here:
    http://tracker.ceph.com/issues/4589



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