k8sutil: Use label max length constant from k8s
k8sutil: Use label max length constant from k8s

Follow-up to #3436:

The validation module has a defined constant for
label value max length. It makes sense to use it
here.

Signed-off-by: Kristoffer Grönlund <8d116301f44f67d979af4b34089c9b5aef6e1b33@suse.com>

rook/rook,rook/rook,rook/nfs,rook/nfs,rook/rook,rook/rook,rook/rook,rook/nfs