rbd: don't assign extent info in rbd_do_request()

In rbd_do_request() there's a sort of last-minute assignment of the
extent offset and length and payload length for read and write
operations.  Move those assignments into the caller (in those spots
that might initiate read or write operations)

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