sched/deadline: Don't use dubious signed bitfields

It doesn't cause a run-time bug, but these bitfields should be unsigned.
When it's signed ->dl_throttled is set to either 0 or -1, instead of
0 and 1 as expected.

The sched.h file is included into tons of places so Sparse generates
a flood of warnings like this:

  ./include/linux/sched.h:477:54: error: dubious one-bit signed bitfield

Reported-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Luca Abeni <luca.abeni@santannapisa.it>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-janitors@vger.kernel.org
Cc: luca abeni <luca.abeni@santannapisa.it>
Link: http://lkml.kernel.org/r/20171013070121.dzcncojuj2f4utij@mwanda


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