sched: check negative err value to safe one level of indent

As it is more common, check err for !0. That allows to safe one level of
indentation and makes the code easier to read. Also, make 'next' variable
global in function as it is used twice.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>