rbd: rename block_name -> object_prefix

In the on-disk image header structure there is a field "block_name"
which represents what we now call the "object prefix" for an rbd
image.  Rename this field "object_prefix" to be consistent with
modern usage.

This appears to be the only remaining vestige of the use of "block"
in symbols that represent objects in the rbd code.

This addresses http://tracker.newdream.net/issues/1761

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