There may have been a benefit to hanging on to the length of an
image name before, but there is really none now.  The only time it's
used is when probing for rbd images, so we can just compute the
length then.
Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NDavid Zafman <david.zafman@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>