Add chmod method to RadosFsInfo

This is added as it is a method common to the subclasses RadosFsFile
and RadosFsDir, which might make it convenient to be used directly
from the base class.
Fix issue when creating a dir with mkdir=true

It was failing when the parent directory was the root because it was
checking not checking for the parent directory's existence before
checking the parent's parent.
test: Cover creating a dir in root with mkdir=true
Don't set the DirCache if the dir's path is a file
Ensure dir is not a file before getting its entries
test: Update DirContents to cover when a dir has a file path