cephfs-shell: refactor do_du()

cephfs-shell: refactor do_du()

Improve code's readability and keep all paths as bytes.
Specifically, define a function within do do_du() to simplify the code
that handles recursion, rename args.dirs to args.paths, get rid of
dir_passed and use args.paths directly instead,  change the default
value of args.dirs to a list with one member that contains current
working directory, rewrite the docstring, and few more similar changes.

Signed-off-by: Rishabh Dave <ridave@redhat.com>