ext4: Use a hash of the topdir directory name for the Orlov parent group

ext4: Use a hash of the topdir directory name for the Orlov parent group

Instead of using a random number to determine the goal parent grop for
the Orlov top directories, use a hash of the directory name.  This
allows for repeatable results when trying to benchmark filesystem
layout algorithms.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>