Root::mkdir_all: should we even return an error for mode=0777?

[cyphar]

This logic was added back when we were trying to defend against being tricked into walking into an attacker-controlled directory, but this was all ripped out in commit ce6226e473e5f72a95eb026293bbce278e55162f ("Root::mkdir_all: remove all mkdir+open verification logic") so maybe we should also remove this validation to match other Root subcommands?