I've finally found some time to look over the per-driver sdev/shost attribute
handling and I'm not so happy with it.  First it adds new writeable
variables to the host templates which is otherwise almost readonly,
the other thing is that it needs per-template procedure calls in the
drivers wheras we have moved away from that.  Also it looks a bit
coplicated :)

I've attached a patch below that makes the attributes handling a lot
simpler.  Details:

 - the shost_attrs and sdev_attrs in the host template are now used
   to store the attributes overriden or added by the LLDD.
 - the midlayer creates those first and then the generic attributes
   that haven't been overridded and the other way around.
 - the host attributes are properly unregistered now and don't leak
   anymore.

Unlike the first patch the attribute inheritance is back.