apparmor: Fix an error code in aafs_create()

We accidentally forgot to set the error code on this path.  It means we
return NULL instead of an error pointer.  I looked through a bunch of
callers and I don't think it really causes a big issue, but the
documentation says we're supposed to return error pointers here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>