We just need a superblock, but we look it up using two different
roots depending on the call site.  Let's just use a superblock
pointer initialized at the outset.

This is mostly for Coccinelle not to choke on my root push up set.
Signed-off-by: NJeff Mahoney <jeffm@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>