sched: rt-group: fixup schedulability constraints calculation

it was only possible to configure the rt-group scheduling parameters
beyond the default value in a very small range.

that's because div64_64() has a different calling convention than
do_div() :/

fix a few untidies while we are here; sysctl_sched_rt_period may overflow
due to that multiplication, so cast to u64 first. Also that RUNTIME_INF
juggling makes little sense although its an effective NOP.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>