rt6_probe_deferred: Do not depend on struct ordering

rt6_probe allocates a struct __rt6_probe_work and schedules a work handler rt6_probe_deferred.
But rt6_probe_deferred kfree's the struct work_struct instead of struct __rt6_probe_work.
This works, because struct work_struct is the first element of struct __rt6_probe_work.

Change it to kfree struct __rt6_probe_work to not implicitly depend on
struct work_struct being the first element.

This does not affect the generated code.

Signed-off-by: Michael Buesch <m@bues.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>