mctp/test: Update refcount checking in route fragment tests

In 99ce45d5, we moved a route refcount decrement from
mctp_do_fragment_route into the caller. This invalidates the assumption
that the route test makes about refcount behaviour, so the route tests
fail.

This change fixes the test case to suit the new refcount behaviour.

Fixes: 99ce45d5 ("mctp: Implement extended addressing")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>