rbd: void data pointers for rbd_obj_method_sync()

Make the inbound and outbound data parameters have void rather than
character type for rbd_obj_method_sync().  This makes it more clear
they don't expect typed data, and eliminates the need for some silly
type casts.

One more unrelated change: define the features buffer used in
_rbd_dev_v2_snap_features() to be a packed data structure.

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