sched,drbd: Convert to sched_set_fifo*()

Because SCHED_FIFO is a broken scheduler model (see previous patches)
take away the priority field, the kernel can't possibly make an
informed decision.

In this case, use fifo_low, because it only cares about being above
SCHED_NORMAL. Effectively changes prio from 2 to 1.

Cc: axboe@kernel.dk
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>