rbd.py: increase parent name size limit

64 characters isn't all that long. 4096 ought to be enough for anyone.

Fixes: #6072
Backport: dumpling, cuttlefish
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit 3c0042cde5a12de0f554a16b227ab437c6254ddd)