For an image object request we will need to know what offset within
the rbd image the request covers.  Record that when the object
request gets created.

Update the I/O error warnings so they use this so what's reported
is more informative.

Rename a local variable to fit the convention used everywhere else.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>