libceph: use (void *) for untyped data in osd ops

Two of the fields defining osd operations are defined using (char *)
while the data they represent are really untyped, not character
strings.  Change them to have type (void *).

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