sched/fair: Propagate 'struct lb_env' usage into find_busiest_group

More function argument passing reduction.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-v66ivjfqdiqdso01lqgqx6qf@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>