sched: Replace first_cpu() with cpumask_first() in ILB nomination code

Stephen Rothwell reported this build warning:

>  kernel/sched.c: In function 'find_new_ilb':
>  kernel/sched.c:4355: warning: passing argument 1 of '__first_cpu' from incompatible pointer type
>
> Possibly caused by commit f711f609
> ("sched: Nominate idle load balancer from a semi-idle package") from
> the sched tree.  Should this call to first_cpu be cpumask_first?

For !(CONFIG_SCHED_MC || CONFIG_SCHED_SMT), find_new_ilb() nominates the
Idle load balancer as the first cpu from the nohz.cpu_mask.

This code uses the older API first_cpu(). Replace it with cpumask_first(),
which is the correct API here.

[ Impact: cleanup, address build warning ]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <20090421031049.GA4140@in.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>