net/garp: fix GID rbtree ordering

The comparison operators were backwards in both garp_attr_lookup and
garp_attr_create, so the entire GID rbtree was in reverse order.
(There was no practical side effect to this though, except that PDUs
were sent with attributes listed in reverse order, which is still
valid by the protocol. This change is only for clarity.)

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>