rust: allow fs driver to initialise new superblocks

They are also allowed to specify the type of superblock keying.

With this change, drivers get a new callback, `fill_super`, that they
need to implement to initialise new superblocks.

In subsequent commits, they'll be able to populate the root with
entries, but for now it's empty.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>

Author: wedsonaf