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>
Merge pull request #764 from ceph/wip-rbd-parent-info

rbd.py: increase parent name size limit

Reviewed-by: Sage Weil <sage@inktank.com>