affs: Replace one-element array with flexible-array member

Replace the deprecated one-element array with a modern flexible-array
member in the struct affs_root_head.

Add a comment that most struct members are not used, but kept as
documentation.

Link: https://github.com/KSPP/linux/issues/79


Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>