sched: Fix up scheduler syscall LTP fails

sched: Fix up scheduler syscall LTP fails

Wu reported LTP failures:

  > ltp.sched_setparam02.1.TFAIL
  > ltp.sched_setparam02.2.TFAIL
  > ltp.sched_setparam02.3.TFAIL
  > ltp.sched_setparam03.1.TFAIL

There were 2 things wrong; firstly __setscheduler() failed on
sched_setparam()'s policy = -1, fix that by reading from p->policy in
that case.

Secondly, getparam() (and getattr()) would still report !0
sched_priority for !FIFO/RR tasks after having been such. So
unconditionally set p->rt_priority.

Reported-by: Fengguang Wu <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Dario Faggioli <[email protected]>
Fixes: d50dde5a10f3 ("sched: Add new scheduler syscalls to support an extended scheduling parameters ABI")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>