sched-fifo-2020-08-04

static priority level knowledge from non-scheduler code.

The three APIs for non-scheduler code to set SCHED_FIFO are:

 - sched_set_fifo()
 - sched_set_fifo_low()
 - sched_set_normal()

These are two FIFO priority levels: default (high), and a 'low' priority level,
plus sched_set_normal() to set the policy back to non-SCHED_FIFO.

Since the changes affect a lot of non-scheduler code, we kept this in a separate
tree.

When merging to the latest upstream tree there's a conflict in drivers/spi/spi.c,
which can be resolved via:

	sched_set_fifo(ctlr->kworker_task);

Signed-off-by: Ingo Molnar <mingo@kernel.org>