rbd: clean up a few dout() calls

There was a dout() call in rbd_do_request() that was reporting
the reporting the offset as the length and vice versa.  While
fixing that I did a quick scan of other dout() calls and fixed
a couple of other minor things.

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