libceph: kill off osd request r_data_in and r_data_out

Finally!  Convert the osd op data pointers into real structures, and
make the switch over to using them instead of having all ops share
the in and/or out data structures in the osd request.

Set up a new function to traverse the set of ops and release any
data associated with them (pages).

This and the patches leading up to it resolve:
    http://tracker.ceph.com/issues/4657



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